So this has been a longstanding issue from what i can tell, but the tracking for certain multuplayer achievements in the MCC stopped some time ago.
Its not every multiplayer achievement, for example the game mode specific ones still track for me, i earned the ctf one a little while ago, but 2 in particular (there may be more) that just dont track are the one for grenade kill medals in matchmaking, and legendary medals. According to the game’s in game medal tracking, i should be somewhere between a quarter of the way done to half way done with the achievement, due to steaktaculars, but my achievement progress (both in the xbox menu and in the in-game tracker) has me somewhere around 10 medals esrned.
I know this issue has effected other players, and im suprised either 343 doesnt know about it (at least from what ive seen online), or that no one has posted about it since they have become so active in fixing the mcc.