I have the same issue. It happens on a lot of "village" combats. The unitgroup "5" (which I use for level 2 or less troops, in order to send them into battle first or to another location so they gain experience quicker) spawns inside the village near the militia. And in my case, as they're inexpienced and usually not in a great number, they get overrun almost immediately before even the cavalry and horse archers can reach them. It happens on different "village" maps in different regions.