This, IMHO, is legally untested at best when it comes to connecting to Proton Mail.
A mail provider that’s required to log IPs saying oh, we can’t log because we put our own VPN in front of the servers isn’t something I think will hold up. That would be quite a loophole for any provider.
I think when Proton tries this the response they’ll get is that they need to log the IP address at the edge of their infrastructure, and their infrastructure pretty clearly includes ProtonVPN.
If you use a VPN other than ProtonVPN with Proton Mail then you should be good, obviously in that case Proton would have no way of knowing your IP.
And if you use ProtonVPN to connect to something other than Proton’s services, I agree Proton couldn’t be forced to log in that case.
It’s important to note that Proton disagrees with me here, and thinks using Proton VPN with ProtonMail would legally protect you. I am also not a lawyer.
Up to you if you want to take the risk.