Turn the Proton killswitch off and use split tunneling, then bind your torrent client to the VPN. This is more reliable than a built-in killswitch.
Or if you're on Linux, spin up one of Binhex's bittorrent+vpn containers. Since you're using Proton, the containers from Binhex will automagically make sure your torrent client is using the random open port Proton picks each time you connect.