so I finished all halo games from ce to 4 on legendary. Here, I thought I would get the achievement, but apparently. It’s only at 97% completion. Any idea why it’s like this?
Try giving it a day or two. Sometimes achievements take a bit to have all the data required for an unlock to sync up. Also, check whether or not Waypoint shows every level of every game as completed. MCC is much more polished than before so it should be recording level completions better, but it’s still possible that’s not the case.
If you see a level as incomplete, try hard resetting your Xbox and restart MCC to force a sync. In case that doesn’t work, you’ll have to play any unfinished levels again.
I checked your MCC service record here in Waypoint, and it appears the only mission you didn’t finish is The Covenant in Halo 3. Looks like you need to redo the mission all over again to unlock the Legend achievement.
Other than that, if you experience any issues involving achievements - go here to report your issue.