Okie dokie, I'm completely sure I've gotten all legendary endings before and seen the on-screen achievement pop up, but it's not showing in the achievements page. I re-did the ending and the achievement didn't pop-up on screen, and it's still not in the achievements page.
Similarly, I got the "You went up against Gisette. You lost" achievement during gameplay, but it's not in my achievements page. Could this be related to the clean slate version? I did wipe everything once, but the other achievements have been showing normally as I get them.