Ah, that makes sense. You're treating the mail as archival; they're treating it as a notification (so of course they don't notify you about the action you already took, since they know you know). And the message sent to you is just the same as the message sent to anyone else when you copy yourself, so it doesn't show the other recipients any more than they see who else you sent it to.

Logical from the frame of how they intended it to be used, but not helpful for how you're trying to use it. This might almost be a feature suggestion.