Examples I currently have are:
Grunt birthday Party Skull Halo CE
Thirsty Grunt on Halo CE
The Megg on Halo CE (shared a photo for proof on my timeline)
Halo 4 mission completion achievements took a fair while to pop too.
Finally - not an achievement bug, but opening terminals in Halo CE, then backing out causing you to sometimes go straight back into the terminal again. Or HaloWaypoint will just not connect (wired ethernet in case you were wondering).
UPDATE Achievement lag seems to be about 20 minutes. Popped a rocket league achievement and lag was 2 minutes - so might be some xbox live issues (Also cannot upload screenshots etc so something going on)
Thanks