Its Blind and Grunt birthday so far
I recently bought the legacy games and playing through halo 3 at the moment. Some skull achievements however dont pop when i collect them.Not saying you’re wrong but just a heads up (at least in Halo 3) there are 3 skulls that don’t have achievements. IWHBYD, Grunt Birthday, and one other. If not those then that’s odd. Let me know which ones and I’ll try it out!
Also some Halo Reach achievements seem to be unlocked sometimes June 2019 although i own them all since last week only and never played reach yet
Okay, Blind and Grunt Birthday don't have a skull achievement attached so that's the issue there. I think they all do on MCC but not OG Halo 3. But, at least it will unlock it to use in campaign!
Edit: From my experience, achievements on Xbox 360 games don't seem to be as finneky as they are on Xbox One. I've had multiple issues with achievement unlocks on Xbox One games but never on a 360 game.