Mullvad does not have port forwarding so it would be normal for your torrent client to be firewalled.
I'm not too sure why you & OP sometimes see it as temporarily connectable when changing/randomizing the incoming port when the VPN service never provided you an open port forward.
Correct.
Yeah that's weird, I don't know if you accidentally found a way to hack Windscribe into temporarily giving you a port forward on their free plan. But otherwise you do need to be a paid member on their Pro account for that feature.
Or it's just going to randomly stop working again.
Is there a way to actually test your port forward within Docker? I'm not familiar enough with that configuration to suggest anything but maybe someone else knows about that. Usually without Docker I'd just start up the torrent client & then use a web browser with any port test website (https://www.canyouseeme.org, https://www.yougetsignal.com/tools/open-ports, etc.). But for Docker not too sure how to go about that.