Have you already got the achievement? As in it popped (bleep bloop and notification on screen) but now its some how unachieved.../locked.
It is very strange. I had it with "Faded" achievement the other day, I had 12 kills exactly and a few in the evac part; the achievement popped but when I went to check out other achievements that needed unlocking it had locked itself up again, so had to achieve it again. I don't know wtf is up but hope you've got it now or near to completing it/getting it resolved.