This is, in fact, happening with a lot of creatures. Shadow Horrors do it occasionally, I found it just after Cory was fiddling with that double-drop bug. However, something in the Nrolav desert did it too. There's a nasty side effect to this bug: the creature you "kill" to start it will not give any drop or EXP yield (no message at all to say it's defeated), but vanishes until you move away. Returning to the square causes the creature to reappear for one hit, then vanish again. Worst of all: if you encounter one of these glitched creatures, you can no longer heal, no matter what you do. You'll get the message, "This person is too far away to appraise". I don't know precisely what's causing this, but all I do know is that it seems that a bug deleted was a greater one created.
Heh heh, that's all right, it's part of the growing pains of an MMORPG...but it destroys any hope I had of levelling on time, as I can't fight in these conditions. It means I essentially will die every time I find a glitched creature, being unable to heal until death sets me free of the bug.