I have just finished the Halo 4 campaign on Legendary solo (07-12-2012), however I did not receive the achievement. I have realised that after applying an update yesterday (06-12-2012) my campaign data has been wiped. After the update I was able to continue the campaign from where I was (the start of mission 7: Composer). However, now I cannot play anything after mission 1 (apart from the last 2 missions, which I have just finished this morning) as it says they are unavailable until I complete the previous mission. It has also lost all of my Co-op campaign data. When I check on Waypoint it acknowledges that I have completed each mission on solo Legendary, but still no achievement.
Same sort of thing happened to me. When i played through co-op legendary, all my friends got “5 for 180G” and the only achievement that i got was for completing mission 8. And i didn’t get the achievement for completing the first 5 episodes of spartan ops 