Master Chief Collection completed on 6/22/15
if anyone happens to see this, PLEASE do not play this game. It is an absolute atrocity and a disgrace to Halo. Achievements won't unlock.
If you disconnect from a mission AT ANY POINT, you entire campaign run is done. You can keep trying, but it'll say your checkpoint is not synced with live.
2 other comments
The only way to fix this is to restart the game entirely, and complete Legendary in one sitting/no turning off game/PC etc. Do not put yourself through this hell.
Through my research, the same goes for Fallout 3 (PC) and GTA IV (PC). Avoid any GFWL at all costs.
And we're off!
Viewed 100 different achievement solutions - TrueAchievements
Self-inflicted paparazzi. - It Takes Two
It takes two to… torture. - It Takes Two