I had completed the entire master chief collection on legendary achievement yesterday 3/2/18 and I did not unlock the legend achievement, on top of that, on way-point it says I havent complete but a few missions but I can prove I beet the whole thing with a few screenshots.Have you checked your career stats and XBL achievement list. I know we saw issues with some achievements not popping immediately. Some came in 30 minutes later, some we exited the game then restarted the CP or restarted the game. Sometimes it takes way point longer to update your profile.
Be sure to run thru the campaign menu lists and check each level difficulty to see if the Legendary icon appears open. If any one of them do not, then you need to run what ever you missed.
It says im 82 or 83% done with the achievement but it shows all the little icons under the highest difficulty completed to be legendary on all of them. and technicly it has been about 9-11 hours since I completed the game.
Ps: I just went back and completed a mission to see if it would update it, it updated halo 4's in like a second but halo 2 wont update at all ( on waypoint)
Pps: I noticed that for the legendary one it said 82% and it was the same for normal, so I went and played one normal mission to see if it would change but it didn't and still did not change on waypoint