Same thing here. Unlocked up to smoke grenade though, every now and then, it will put me back to 0 xp for grenades.
This leaves me with the first grenade unlocked only and when I get more xp the amount never increases for the duration of gaming session.
They have come back and dissapeared a few times now.
Playing the Xbox 360 version and I do hope they fix this with the next multiplayer update.
Also just today it set back my amount of xp in general for my assault kit. Earlier (about 2 hours earlier) I had earned the medic's bag at 101000 xp. When I logged back in I was back at 97000 ish where I had started previously and had lost my promotion to level 11. Not going to lie, the multiplayer is fun though completely full of bugs and things that need fixing such as invincible helicopters and this grenade/unlocks deal.
