Using qBittorrent (v4.6.5) and Mullvad VPN (post their removal of one’s ability to port-forward) I am able to seed a torrent that I have downloaded; however, if I create a torrent locally and try to share it, it will not seed. It’s only when I disable the VPN that it will seed. I have not port-forwarded manually — qBittorrent is using UPnP (which I assume Mullvad VPN blocks).

I don’t understand the difference, though. Why am I be able to seed an established torrent, but not a new one? Afaik, both cases would need to accept incoming connections, which would require a forwarded port.