On my Discourse install, running
v1.7.0.beta7 +215, the following system message is being sent to the wrong user:
The user in question was the first user created when Discourse was installed. Later, I demoted this user to moderator and added a separate admin user (because I wanted to use a stronger password for admin functions).
The admin user does not receive this system message directly or through the admin inbox, but the first user, who used to be an admin and is now a moderator, does.