This isn't so much as a "I didn't get the achievement" post, as it is a "I didn't get the "pop" notification for it and my achievement tracker said it still was locked."
It did fix itself though, so now I just want to tell you what happened, incase if I could help a fellow gamer out.
This is probably something I should take up to whoever does the achievements/achievement recognition software on Xbox entirely, but I never really had any achievement problems for mcc (I only did the campaign-focused ones, until Reach fell)
These achievements were "Dim Mak" and "Requiescat in Pace"
I got the needed medals to get both, but dim mak was stuck on 99% and the assassin one was 80%, after a few games of sneaking around and getting more medals (even though it should have been complete), I got tired and went to waypoint to check the achievements were there or not, thankfully they were, so I checked back on my xbox to see whether it had updated or not, it had not. So I closed mcc and opened up Halo 5 for the achievement tracker so it can load a different set of achievements, then went back to mcc. No dice, then I went to check all of my achievements on my profile, and there it was, then I went back to the achievement tracker and this visual error was fixed! I hope this can help a few people out!
P.S. Are other people experiencing the achievement tab (for mcc) to be a bit sluggish than before, it seems that after the Reach update, at least for me, I couldn't scroll down my long list quickly, (I have 135 left)) sometimes I would have to wait a while for the tracker to catch up to my inputs. It's not like this for any other Xbox games.
EDIT: Dunno whether someone at 343 did it or not, but my achievement scrolling problem is gone, either that or because I got a lot of achievements just by playing reach, the number of total achievements left to display has decreased and it’s a lesser load on the console or something.