Same issue here. It's a relief to read I'm not the only person having this exact issue.

This is still broken in MacOS 12.2

Maybe I should rephrase the question. In PGP/GPG the signature and the encrypted message are usually embedded as attachments in the email. This way, a recipient that doesn't have PGP/GPG installed (anymore) will see an empty email with an encrypted.asc file instead of the encrypted gibberish in the body of the email. I'd like to do the same as PGP/GPG and store the signature and the ciphertext in an attachment. How do I do that?