PGP signatures on aliases
With the main account enabled for PGP signing of outgoing messages, any aliases on that account (who don't have their own PGP keys) are signed with the master account's key. So the receiver gets a valid key, but for a different email address to the one from which the message was sent.
I observed this on Evolution 3.28.1-2, on Ubuntu 18.04.1 LTS 64-bit