It feels to me that when I can't loot an NPC, that I'm not completely out of combat or something. Resetting the UI doesn't seem to work either.
As luck would have it, usually there is another mob near by that I can lure close enough that when I loot it, I get the loot from the other NPC also.
But yes...it happens to me also, usually towards the end of a long session of playing, which might be part of the "Memory Leak" symptoms being discussed else where on the forums.
Five months in, I think we can stop referring to them as 'beta' bugs.
Besides, this one is not that uncommon in games. My personal workaround is to stop being so twitchy with the knockbacks when an NPCs death is imminent