Easy way to test Pause/stop all your torrents > shut off the VPN > go out to ipleak.net > use the torrent address detection section and use the magnet link. If done right you should never get a file list from the magnet link/the torrent won’t start because qbit is trying to use the down VPN interface.

In qbittorrent > settings (gear icon) > advanced settings. There will be a option that lets you select a network interface. You should see your Proton VPN interface in the list, select it and you should be set. I would recommend checking the app after patching to make sure the settings persists.