Assuming that you trust what Proton says, when they receive a (possibly unencrypted) message they re-encrypt it with your key as soon as possible and they don’t log the content. So, after that point, they (or anyone else) can’t read the email contents. If it was also encrypted in transit, then there’s only a small window inside their email processing system where the plaintext was passed from one encryption to the other. It’s only decrypted again in your browser or proton mail app with the key that only you have. It’s not bulletproof, but it’s better than most providers.
A quick fix might be to disable any ipv6 addresses if you don’t specifically need them. The vpn /could/ be ipv4 only, which /could/ leave your ipv6 free to leak or make ipv6 dns requests.