If that is indeed the case you should report this issue with as much detail as possible to the Proton team, because it seems like qBit is behaving propperly and there’s some portion of Proton virtual adapter that is failing here.
I use Proton Vpn as well, but I have a custom wireguard interface & server switching script via their API that doesn’t run into the same issue you’re describing. So the issue must lie somewhere in the Linux GUI app.
Do you get the same issue if you try using an openvpn or wireguard config generated from logging into the proton vpn website? or maybe just from the CLI version of the app?