Funny one. What I think happened: the reply hit your email-in address, and because the notification address was in the Cc, incoming email processing treated it like any other new address on the thread and staged a user for it.
No harm in the short term, but I’d suspend that staged user (or anonymize it) so nobody can @mention it or add it to a PM by accident. Agree that the site’s own outgoing addresses should be skipped by default when staging users.