This is a fairly odd ‘bug’, in which my progress on this achievement has suddenly stopped at 50/500 nade kills. I am unsure on how this has happened but according to my stats on Matchmaking, I have 645 grenade kills, so this achivement should of popped a while ago now.
I also tested this by aiming to get nade kills in a few games and as suspected, no additonal progress was made.
Anyone else encountered this bug?