I'm not up on my signal protocol but using PGP, sending encrypted messages to the wrong person would result in them getting an encrypted message they can not de

I'm not up on my signal protocol but using PGP, sending encrypted messages to the wrong person would result in them getting an encrypted message they can not decipher. If a bug in signal allows a third party to decrypt a message intended for another person, does that means that signal servers see plaintext messages?