NGINX reverse proxy does not handle multipart form for larger requests

Question

I have an NGINX configuration that sends FormData to another backend service. When the formdata uses only one file, it sends the data to the backend without a problem. However, once I add more data and files, and the request gets larger.. NGINX can't forward the request to the backend.

I've tried turning proxy_request_buffering off, but that did not work.

I tried using the below code:

client_max_body_size 100M;
client_body_buffer_size 100M;
client_body_in_file_only clean;
client_body_in_single_buffer on;

location /uri/ {
    proxy_pass http://proxy;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection 'upgrade';
    proxy_set_header Host $host;
    proxy_cache_bypass $http_upgrade;
    proxy_connect_timeout 600;
    proxy_send_timeout 600;
    proxy_read_timeout 600;
    proxy_buffer_size 128K;
}