Using a VPN that is blocking port-forwarding, why would an established torrent be able to seed, but a new one wouldn't?
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.
It might be continuing to use the prior network adapter foe the existing connection, assuming your VPN installs itself as a virtual adapter? Might beuave differently if you restart.
I don't think that's the case, as I have qBittorrent manually set to only use Mullvad VPN's network adapter (Preferences>Advanced>Network interface>wg0-mullvad).