I had this issue in the normal game and in the beta where if an enemy is standing on a corpse then they are untargetable as it is trying to use abilities on the corpse with the message "Better choose a different target: [Corpse #] is dead or dying".
I have also had a few enemies stepping into hexes that are already occupied by other enemies and allies but that seems to happen rarely and I don't know what triggers it at all.