nginx is not able to detect when service goes down and comes up

We have an UI application deployed using Nginx.

when any services go down, the response changes to 504 gateway timeout, which is fine, however when the service comes back up this is not detected and 504 gateway timeout is still returned to the UI.

I have tried a bunch of Nginx config but none seems to work. some of the configs tried are:

location ~ ^/(hub/note|rest/api/note) {
    proxy_pass http://localhost:8011;
    proxy_buffering off;
    proxy_http_version 1.1;
    proxy_set_header Host $http_host;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
    proxy_cache_bypass $http_pragma;
    proxy_no_cache $http_pragma;
    proxy_cache off;
    proxy_cache_use_stale off;
    proxy_connect_timeout 180s;
    proxy_read_timeout 180s;
    proxy_send_timeout 180s;
}

did someone face a similar issue?

UPDATE: if I reload Nginx using nginx -s reload, connections are successful.