Hello! I have kept seeing this bug with Haunted Minions, and thought I would share it.
I made a new wizard to play through the new wiz experience (which I think is fantastic!). My wizard has the first tier Ambrose storm wand (Headmasters pack), which for my wizard does 53 damage points. Several times when battling Haunted Minions, I've done some kind of attack and the HM has 53 health points left. I'll then do a wand hit at 53 attack points and the Haunted Minion does the defeat animation and vanishes. However, then the battle goes right to the next round and the Haunted Minion has 1 health point left, but the HM itself is not shown. It's just an empty cicle on the battle board. When I attack it again it actually does "die".
Just to let you know, this is not me miscalculating! I'm 100 % sure the minion had 53 points left and my wand hit did 53 points. This happened at least two times, both in Triton Avenue and on some of the other "streets" in Wizard City.