A.K.A a bugRequiel said:2.8 doesn't usually bug, certainly I've never seen it bug ever.
However there is a fairly common way in which you may not get credit. If you're using a very high damage class such as a nuker, you may kill him too fast.
When the mob reaches about 10% life a flag goes up that will grant you credit when he dies. If he drops from >10% to 0 in one shot, this flag doesn't get a chance to be set and so you don't get credit.
Nuke him down to about 15% and then finish him off with smaller nukes to be sure. Remember you can step back off the pad to heal or rest at any point.