I have recently been trying to complete Halo 4 on solo Legendary to get the achievement for it (this is also the first time I have played through Halo 4 in MCC, so I would be getting all completion achievements). After playing the mission Requiem, I did not get the achievement for completing it, though the game has record of me finishing the level on legendary. After just now finishing the whole game solo Legendary, I did not get any achievements for completing it, for any difficulty. I assume this is because I did not get the achievement for completing Requiem, but I would rather not play it again on Legendary, cause I had a lot of trouble the first time through haha. Any help would be greatly appreciated.
Your assumption would appear correct. It looks like the game bugged out tracking your requiem run through. You have to replay it on legendary and hope it tracks it a second time around. 
sorry mate
Chdck out some guides on YouTube. Those usually help me get through a difficult section. (Especially for LASO)
Thanks man! I’ll drop an update when I do it