So I imagine veterans here have seen plenty of topics of people completing a LASO playlist and not having the achievement unlock for them. That’s the situation I’m in with Halo Reach. I have 11/11 completed on the halo reach playlist and unfortunately haven’t had the achievement pop for me 
I submitted a support ticket on Halo support just a little over a week ago but it doesn’t seem like any progress has been made, sadly. If I have to do the playlist again my buddy said he’d gladly do it even though he already got the achievement (when I also should have gotten it with him lol). Wanted to try first before doing the playlist over again to see if anyone had a save on LASO at the pillar of autumn or hell even close to the end. Any advice or help is appreciated!
try to download again your profile on your xbox, if not you can go in the menu of your xbox, go on the game halo MCC and press select, then you click on manage the game, you should see afterwards some save data with your gamertag next to it, you can delete them but be careful to choose ( delete on this xbox only ) then when it’s done, you launch MCC, your account will resynchronize, and there your success should certainly be unlocked.
> 2533274860712851;2:
> try to download again your profile on your xbox, if not you can go in the menu of your xbox, go on the game halo MCC and press select, then you click on manage the game, you should see afterwards some save data with your gamertag next to it, you can delete them but be careful to choose ( delete on this xbox only ) then when it’s done, you launch MCC, your account will resynchronize, and there your success should certainly be unlocked.
Unfortunately I only own MCC on PC/Steam and just use my Xbox account to link it to my steam account. Do you know if by owning the game on PC I would be able to do the steps you suggested on Xbox?