I don’t want to get anyone’s hopes up, but after I completed H2A on legendary and went back to the “missions” menu under Halo 2 all of the achievements that hadn’t popped began to pop in succession. Then, when I quit the game from the xbox home screen and went back in, all the avatars and nameplates that hadn’t shown up because the achievements hadn’t popped were finally there.
Give it a shot, might work for you.