why doesn't nginx load balance not work? how fixed it?

Question

the nginx config below the server 28081 always return 502, i config the proxy_next_upstream, but the request does not passed to next node (28082) nginx version:1.19.3

 upstream boot-server {
    server 172.17.112.90:28081 weight=9;
    server 172.17.112.90:28082 weight=1;
 }


server {
        listen 80 ;
        server_name  some.domain.com;
        root    /home/
        location /v3/ {
                proxy_pass http://boot-server;
                proxy_next_upstream http_502 non_idempotent;
                proxy_read_timeout      7200;
                proxy_connect_timeout   5;
                proxy_set_header        Host                $Host;
                proxy_set_header        X-Forwarded-For     $remote_addr;
                proxy_set_header        Content-Type        "application/json";
        }
    }