Are you using OpenVPN or Wireguard configs?

For Wireguard I had to slightly modify the commands to open a port to:

natpmpc -g 10.2.0.1 -a 0 0 tcp 60

natpmpc -g 10.2.0.1 -a 0 0 udp 60

while true ; do date ; natpmpc -g 10.2.0.1 -a 0 0 tcp 60 && natpmpc -g 10.2.0.1 -a 0 0 udp 60 || { echo -e "ERROR with natpmpc command \a" ; break ; } ; sleep 45 ; done

I haven't used ProtonVPN, but for PIA the port never showed as open until traffic was going through it.

Once I put the port into qbittorrent and downloaded for a couple of minutes, the port then showed as open for port test tools.

You must check that the client you're using is trying to use the port that ProtonVPN actually forwards