You must have made a mistake somewhere, as once I close the Proton VPN client (actual "close", as in the app is not running anymore) my qBit does stop downloading (just tested), although the rest of the system still has access to the network (must have something to do with how apps are installed on Linux, I guess).
In my qBit settings > advanced > network interface I read "tun0", and that's what I think was suggested in the official setup guide on Proton. What do you have?
Same approach, especially now that I upgraded my storage. Got a ratio of 135 on one file, no idea how or why though