Same problem occured to me, although I played earlier this day with no issues. Happened in act 2 turn 23. I remember solving the problem
with my generals destroying undead orphanage
then I set my units on strategic map and saved the game. When I loaded it later and proceeded to next turn, battle was already bugged. There were several battles on the map - all of them with the same issue. My copy of the game is from steam if its relevant.
