“Therefore, all user-applied signatures will be broken. Tough luck, bucko, we’re the SECURE email company, you’ll upload your private key to our servers and you’ll like it!”
Proton Bridge mangles the MIME structure of any message you sign yourself with PGP, so the signature fails to verify. The bug reports on GitHub go back to the start. There is no setting to tell Proton to leave a signed message alone. The secure email company only trusts the keys it holds.