I finally got this bug today, not just once but many times several hours apart, all times it was fighting Angry Grouks NW of Sslanis. So I am thinking I can duplicate it at this spot again. It was very "heavy" and affected at least 25%-50% of the mobs (they'd be dead but still show some health and standing. You get XP but could not loot or target next-nearest enemy etc).
If you want me to try to duplicate next time you are on, let me know and I will let you know, that way you don't have to go there until I can confirm it is still happening with those mobs.
I do not think it is lag related as it even happened at 4AM when only three people or so on the server. I also get in this same area, odd occasional hits of damage against my health for no apparent reason.