I'm having the same problem as xGODx, well sort of.
I beat the game all the way through on Legendary solo without stopping, obviously shut my xbox off to sleep, eat, and socialize (which was all done after beating a mission).
Anywho, so I have all of the achievements and when I go through my campaign missions to look at them it shows that I've beat them all on Legendary solo.
Now my problem is that one of them, the Infinity mission, isn't showing up as done in this location.
commendations >> player >> campaign >> Infinity
Anyway to fix this or will I have to go through again and beat it?