So I hit 50 in multiplayer, maxed out the exp and unlocked the CELL skins, rebooted my suit and now i'm about 31 but I still have a missing mystery dog tag to unlock. Did I miss something or did it bug out?

Bug ofc. I was missing 6 but than on lev 49 I got back the remaning so I was lucky. Maybe the next patch will fix it for you.