I’m using gluetun with AirVPN and my transmission containers’s connection to peers isn’t great. I have port forwarding enabled (verified working using the test button in transmission). But I can connect to 0 or 1 peer out of 12 on a specific torrent, and without the VPN I can immediately connect to 6 peers.

Did you do anything tricky with your container stack that isn’t part of the gluetun guide?