Just as the title states.
As of today I have completed the “Devastating” achievement requirements. My Halo Waypoint service record shows times well under 3 hours for Halo 3 (solo) and as far as I can tell this should have unlocked it. I have gone through every level (except Cortana) in the last few days, with The Covenant and Halo just today.
I have beaten Halo 3 on Legendary (both solo and co-op) before and my first thoughts are that over the course of all the updates something has gone wrong. I haven’t played in quite a while and so it would seem possible that some times have become invalid due to changes in updates (keep in mind I haven’t done Cortana yet recently), but I have no way of checking this myself.
I have found many previous topics surrounding both LASO and other sub-three-hour achievements, so I’m hesitant to attempt those in the future.
Any info regarding what I can try next is appreciated.
Some noteworthy info: I was streaming the game to my windows 10 pc through the xbox app, but this never game me any issues besides input latency sometimes.
EDIT: forgot to mention that it does not show up in the xbox achievements list as “100%, unlocking…” but simply as “locked”. I saw this being suggested in a stickied post from 2014 which suggests this will be fixed in a future update… but I doubt it’s the same issue.
I originally had the same issue as well when doing this achievement. I played through the Halo 3 Legendary playlist in under 3 hours but the Devastating achievement didn’t pop up at first. I restarted my Xbox and once I booted up MCC again the achievement unlocked. So if you haven’t already, try restarting or maybe even hard resetting your Xbox and then loading up the game again.
> 2533274812864671;2:
> I originally had the same issue as well when doing this achievement. I played through the Halo 3 Legendary playlist in under 3 hours but the Devastating achievement didn’t pop up at first. I restarted my Xbox and once I booted up MCC again the achievement unlocked. So if you haven’t already, try restarting or maybe even hard resetting your Xbox and then loading up the game again.
Well… I thought I tried that already and it didn’t work but I turned it on again today to try and beat my time on Cortana and the moment I entered the Halo 3 mission select screen it unlocked. I’m confused why this happened but I’m glad it did. Thanks for the info though.
Ayyy! Happy it finally unlocked
this just happened with me but for ODST’s campaign… i did it a long time ago and it just popped up now when i spontaneously decided to launch MCC to check out the signs on valhalla for bungie day 
I play MCC pretty often and did that legendary odst campaign months ago… does make me uneasy that achievements like that may be glitched. especially with some people trying to 100% MCC