Why is my server getting connections from its own IP?

The reason you will see connections from your servers IP is usually due to using a reverse proxy. Over the years there has been changes to how this can be resolved.

The original way was using the X-Forwarded-For header. Then there was a standardised way of doing it with RFC 7239 which is called Forwarded. Here is an article from NGINX explaining this.

The final option is to use the proxy protocol. HERE is the official documentation around this from NGINX.

To understand this all more in depth here is a fantastic post explaining it all.

This is an educated guess.

You have a WordPress plugin that somehow views files uploaded to the phpBB forum you have. Internally it downloads the file from phpBB, and then presents it via some WordPress -side page.