I don't want to revoke a key to test but I'm pretty sure that just sets the port in the Endpoint part of the WireGuard config file. (the port you use to connect, for if the regular one is blocked). Are you sure your service behind Mullvad is accepting incoming connections?

Is it a torrent client, by any chance? Those can still work without port forwarding, if the swarm member you're sharing data with (regardless of direction) has an open port on their side.

Try creating a new torrent with some random file, seeding it from a Mullvad device and downloading it from a different Mullvad device. That should only work if you have port forwarding set up (or if you're not actually going through Mullvad - you will see that by the peer IP in the torrent client).

That custom port on the WireGuard config page is not the place where you'd configure port forwarding; that's not what that is. They had a separate port forwarding page for configuring city ports which is now gone. But you say you have it working. My guess is that you're just misremembering where the configuration is, and that Mullvad hasn't removed existing port forwards yet like they said they would.

The "custom port" option in the config creator just sets the endpoint port to use for Wireguard. It has nothing to do with port forwarding.