I have had this happen a lot on boss fights where when I defeated the boss, like over kill or whatever, they are still not dead, as if they are able to come back with 1,000 health. I don't know what is going on but I know it is not supposed to happen.
Before today I had never experienced this... but now it has happened to me twice in just a couple hours! First time was helping someone with a dungeon in Azteca. I don't remember the boss's name, but I do know I overkilled, yet the boss came back next round (although invisible, so his pips were visible but both his health and actual form weren't) with 4k health. Second time was while fighting D'viss in Empyrea -- he has 12k health, my hit did 14k, next round he comes back (also invisible) with 2k health. So odd...