Certain matchmaking achievements have stopped tracking for me, specifically ones like Catch This (500 grenade kills), Domination (100 legendary medals) and Nice Work Dawg (2500 assists) among others are just stuck in place.
Hey OP, if you’re still having issues with achievements please make sure and log the details in the feedback thread here:
https://www.halowaypoint.com/en-us/forums/84ad72a8b51847978545f685f651fc15/topics/achievement-progress-unlock-reports/d3a61929-3917-4224-b723-be59860816e8/posts