I am just disgusted by this.
Last night I was on the last mission for Halo 3, playing through the MCC for the first time. For the past month no problems with Halo: CE and Halo 2 and Halo 3 so far. Last night all of a sudden - the infamous “dlc not installed” error.
I reinstalled twice, through the Xbox app and the Microsoft Store.
I “installed” every single game through the Microsoft Store.
No luck.
I just created a ticket on Halo Waypoint but I have little hope. Microsoft just lost a subscriber, and no I am not buying this on Steam.
From what I’ve gathered reading the forums all day is that the Windows 11 release broke the game.
GG Microsoft, you’ve managed to tarnish Halo in my eyes forever.
Huh, neat.
That’s quite an annoying issue, and I can feel the frustration but an issue like this will most likely get resolved sooner than later. No need to decry the MCC as garbage for something that OS devs broke vs the game devs.
DRAMATIC FINISH!!!
And that is why I would rather be a console gamer…less stuff that can break the game… 
Hey folks, this is a wider spread issue with MCC on Windows Store right now, please see this existing topic. I edited a potential fix into the first post that might be worth trying, also always good to view the last few posts as well
https://www.halowaypoint.com/en-us/forums/85d779d52cfd46918b4f8b638f2e3c7b/topics/mcc-pc—missing-halo-___-dlc/c51c3be6-dfe2-4c96-a77a-ffef330e2061/posts