While I see the achievement is indeed unlocked on xbox.com the game's GS and achievements unlocked have not updated for the last achievement meaning our scanners too cannot see this change.
There are some more details in my post here
[NAB] Assassin's Creed Liberation HD glitched achievements unfortunately for you though you cannot get another achievement to force the game to update and honestly I don't know what to suggest. The game should eventually sync... I hope.