just in case – if you can not find the reason in the log (the last battle in the session, I had to retreat of it, because I could not finish it)
2 necromancer was at some point, I saw the two of them become one hex. When I killed first the second necromancer stayed at this hex and become unavailable for the attack (the invalid target) even a dog, specially released me at the end of the battle could not kill him – just running around