Hello there, tirijalito here!
So, as I specified in the title, I just finished the campaign in legendary difficulty (finally) and I spent some time without internet, no achievements, but who cares. Today I did the last episodes with internet and, to my surprise, the game says I did not end the campaign at all. WHAT?
What am I supposed to do now?
Thanks!
Well,probably the achievement is missing because you completed that missions offline.
Check your profile (https://www.halowaypoint.com/es-es/games/halo-the-master-chief-collection/windows/service-records/players/tirijalito) and find out what mission is missing. Seems like you had all completed but for some reason “Exodus” didn’t has score,maybe you should replay that one.