Its a bug where they havent technically "entered" the field yet. I've had that bug happen to me where my and the AI's reinforcements spawned in at hte same entry point and both sides got stuck. One of their line inf units manage to get half of their units onto the field but the flag bearer/id was still stuck at the spawn point, so what resulted is a unit that cant do anything nor be targeted except via bayonets.

Were you able to win the battle? Cuz i wasnt since couldn't beat "everyone" on the field.