Nginx proxy serves content from wrong subdomain

Question

I'm using jwilder's nginx proxy (https://github.com/jwilder/nginx-proxy) to route traffic for several dockerized applications. Each docker application runs its own Nginx server with PHP, MySQL database and has SSL certificate configured via Let's Encrypt. If only one of the applications is running at the time, everything works OK. The problem is when running all applications at once over proxy mentioned before. For example, when I access the first domain at app1.example.com, sometimes it loads perfectly fine, but other times it loads the content of the second domain at app2.example.com. It does not redirect, the URL is right, only content gets mixed up.

Thank you for any help!

Below is the contents (domains and IP addresses are edited) of the proxy's config file:

# If we receive X-Forwarded-Proto, pass it through; otherwise, pass along the
# scheme used to connect to this server
map $http_x_forwarded_proto $proxy_x_forwarded_proto {
  default $http_x_forwarded_proto;
  ''      $scheme;
}
# If we receive X-Forwarded-Port, pass it through; otherwise, pass along the
# server port the client connected to
map $http_x_forwarded_port $proxy_x_forwarded_port {
  default $http_x_forwarded_port;
  ''      $server_port;
}
# If we receive Upgrade, set Connection to "upgrade"; otherwise, delete any
# Connection header that may have been passed to this server
map $http_upgrade $proxy_connection {
  default upgrade;
  '' close;
}
# Apply fix for very long server names
server_names_hash_bucket_size 128;
# Default dhparam
ssl_dhparam /etc/nginx/dhparam/dhparam.pem;
# Set appropriate X-Forwarded-Ssl header
map $scheme $proxy_x_forwarded_ssl {
  default off;
  https on;
}
gzip_types text/plain text/css application/javascript application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript;
log_format vhost '$host $remote_addr - $remote_user [$time_local] '
                 '"$request" $status $body_bytes_sent '
                 '"$http_referer" "$http_user_agent"';
access_log off;
resolver 127.0.0.11;
# HTTP 1.1 support
proxy_http_version 1.1;
proxy_buffering off;
proxy_set_header Host $http_host;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection $proxy_connection;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $proxy_x_forwarded_proto;
proxy_set_header X-Forwarded-Ssl $proxy_x_forwarded_ssl;
proxy_set_header X-Forwarded-Port $proxy_x_forwarded_port;
# Mitigate httpoxy attack (see README for details)
proxy_set_header Proxy "";
server {
    server_name _; # This is just an invalid value which will never trigger on a real hostname.
    listen 80;
    access_log /var/log/nginx/access.log vhost;
    return 503;
}
# app.example.com
upstream app.example.com {
                ## Can be connected with "nginx-proxy" network
            server 192.168.240.11:443;
}
server {
    server_name app.example.com;
    listen 80 ;
    access_log /var/log/nginx/access.log vhost;
    return 301 https://$host$request_uri;
}
server {
    server_name app.example.com;
    listen 443 ssl http2 ;
    access_log /var/log/nginx/access.log vhost;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
    ssl_ciphers 'ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:!DSS';
    ssl_prefer_server_ciphers on;
    ssl_session_timeout 5m;
    ssl_session_cache shared:SSL:50m;
    ssl_session_tickets off;
    ssl_certificate /etc/nginx/certs/app.example.com.crt;
    ssl_certificate_key /etc/nginx/certs/app.example.com.key;
    add_header Strict-Transport-Security "max-age=31536000" always;
    location / {
        proxy_pass https://app.example.com;
    }
}
# app1.example.com
upstream app1.example.com {
                ## Can be connected with "nginx-proxy" network
            server 192.168.240.10:443;
}
server {
    server_name app1.example.com;
    listen 80 ;
    access_log /var/log/nginx/access.log vhost;
    return 301 https://$host$request_uri;
}
server {
    server_name app1.example.com;
    listen 443 ssl http2 ;
    access_log /var/log/nginx/access.log vhost;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
    ssl_ciphers 'ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:!DSS';
    ssl_prefer_server_ciphers on;
    ssl_session_timeout 5m;
    ssl_session_cache shared:SSL:50m;
    ssl_session_tickets off;
    ssl_certificate /etc/nginx/certs/app1.example.com.crt;
    ssl_certificate_key /etc/nginx/certs/app1.example.com.key;
    add_header Strict-Transport-Security "max-age=31536000" always;
    location / {
        proxy_pass https://app1.example.com;
    }
}
# app2.example.com
upstream app2.example.com {
                ## Can be connected with "nginx-proxy" network
            server 192.168.240.5:443;
}
server {
    server_name app2.example.com;
    listen 80 ;
    access_log /var/log/nginx/access.log vhost;
    return 301 https://$host$request_uri;
}
server {
    server_name app2.example.com;
    listen 443 ssl http2 ;
    access_log /var/log/nginx/access.log vhost;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
    ssl_ciphers 'ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:!DSS';
    ssl_prefer_server_ciphers on;
    ssl_session_timeout 5m;
    ssl_session_cache shared:SSL:50m;
    ssl_session_tickets off;
    ssl_certificate /etc/nginx/certs/app2.example.com.crt;
    ssl_certificate_key /etc/nginx/certs/app2.example.com.key;
    add_header Strict-Transport-Security "max-age=31536000" always;
    location / {
        proxy_pass https://app2.example.com;
    }
}
# app3.example.com
upstream app3.example.com {
                ## Can be connected with "nginx-proxy" network
            # hifiautofeed_web_1
            server 192.168.240.17:443;
}
server {
    server_name app3.example.com;
    listen 80 ;
    access_log /var/log/nginx/access.log vhost;
    return 301 https://$host$request_uri;
}
server {
    server_name app3.example.com;
    listen 443 ssl http2 ;
    access_log /var/log/nginx/access.log vhost;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
    ssl_ciphers 'ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:!DSS';
    ssl_prefer_server_ciphers on;
    ssl_session_timeout 5m;
    ssl_session_cache shared:SSL:50m;
    ssl_session_tickets off;
    ssl_certificate /etc/nginx/certs/app3.example.com.crt;
    ssl_certificate_key /etc/nginx/certs/app3.example.com.key;
    add_header Strict-Transport-Security "max-age=31536000" always;
    location / {
        proxy_pass https://app3.example.com;
    }
}
# www.app.example.com
upstream www.app.example.com {
                ## Can be connected with "nginx-proxy" network
            # alttagcmsapp_web_1
            server 192.168.240.11:443;
}
server {
    server_name www.app.example.com;
    listen 80 ;
    access_log /var/log/nginx/access.log vhost;
    return 301 https://$host$request_uri;
}
server {
    server_name www.app.example.com;
    listen 443 ssl http2 ;
    access_log /var/log/nginx/access.log vhost;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
    ssl_ciphers 'ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:!DSS';
    ssl_prefer_server_ciphers on;
    ssl_session_timeout 5m;
    ssl_session_cache shared:SSL:50m;
    ssl_session_tickets off;
    ssl_certificate /etc/nginx/certs/app.example.com.crt;
    ssl_certificate_key /etc/nginx/certs/app.example.com.key;
    add_header Strict-Transport-Security "max-age=31536000" always;
    location / {
        proxy_pass https://www.app.example.com;
    }
}
# www.app1.example.com
upstream www.app1.example.com {
                ## Can be connected with "nginx-proxy" network
            # elstrote_web_1
            server 192.168.240.10:443;
}
server {
    server_name www.app1.example.com;
    listen 80 ;
    access_log /var/log/nginx/access.log vhost;
    return 301 https://$host$request_uri;
}
server {
    server_name www.app1.example.com;
    listen 443 ssl http2 ;
    access_log /var/log/nginx/access.log vhost;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
    ssl_ciphers 'ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:!DSS';
    ssl_prefer_server_ciphers on;
    ssl_session_timeout 5m;
    ssl_session_cache shared:SSL:50m;
    ssl_session_tickets off;
    ssl_certificate /etc/nginx/certs/app1.example.com.crt;
    ssl_certificate_key /etc/nginx/certs/app1.example.com.key;
    add_header Strict-Transport-Security "max-age=31536000" always;
    location / {
        proxy_pass https://www.app1.example.com;
    }
}
# www.app2.example.com
upstream www.app2.example.com {
                ## Can be connected with "nginx-proxy" network
            # riverstream_web_1
            server 192.168.240.5:443;
}
server {
    server_name www.app2.example.com;
    listen 80 ;
    access_log /var/log/nginx/access.log vhost;
    return 301 https://$host$request_uri;
}
server {
    server_name www.app2.example.com;
    listen 443 ssl http2 ;
    access_log /var/log/nginx/access.log vhost;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
    ssl_ciphers 'ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:!DSS';
    ssl_prefer_server_ciphers on;
    ssl_session_timeout 5m;
    ssl_session_cache shared:SSL:50m;
    ssl_session_tickets off;
    ssl_certificate /etc/nginx/certs/app2.example.com.crt;
    ssl_certificate_key /etc/nginx/certs/app2.example.com.key;
    add_header Strict-Transport-Security "max-age=31536000" always;
    location / {
        proxy_pass https://www.app2.example.com;
    }
}