Good morning, This happened to me twice so I figured I would point it out.
When I hit for exactly the amount of life in an enemy (i.e., monster has 500 hp, my hit causes exactly 500 hp damage), the monster did its "defeated dance" and disappeared...but then the next round there was a blank space with pips and 1 hp. One more hit defeated the enemy.
I've seen this in the deckathalon events, as well as even some PVE fights (though I can't remember which one in particular). It's also (for me) not just if you do the exact amount of damage, but also if you overkill sometimes too. It seems to be only certain creatures and I'm not sure why.
