Its pretty weird, being specifically bound to tun0 and still having your IP show after disconnecting, makes it seem like there is something else running tun0 like openVPN or Wireguard without a kill switch enabled and they’re just not connected to a VPN, if you’ve ever used them?
You could probably test this if you exit qbittorrent completely and disconnect from your VPN then open qbittorrent again and connect to another network interface in qbittorrent like wlan0 and save it. Then reopen that setting and see if tun0 is still an option. I don’t think it should show up, if it does something is keeping tun0 active.
Either way, I would suggest uninstalling ProtonVPN v3 and v4 and reinstalling v4, maybe something is getting mixed up with v3 and v4 files?