You need to make sure the reverse proxy forwards all the headers to the FileRun server. In other words, your reverse proxy must be fully transparent.

Here is an Nginx configuration example:

proxy_set_header Host $host;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header X-Forwarded-Protocol $scheme;
proxy_set_header X-Forwarded-Port $server_port;

If the custom port number is not being forwarded, you might want to try to change the last line like this:

proxy_set_header X-Forwarded-Port $port;