I've noticed that many times. When a battlefield has a heavy terrain (mountains, thick forests, big roots, rivers) retreating units often get stuck. The enemy is crused, the battle outcome is obvious, but retreating forces cannot reach the border of a map and one needs to spend time hunting down several units before one could proceed. It is especially like this in the vincity of Moria: there is a scene with a river, crushed enemies run into it and get stuck in one line not being able to cross the river.

I don't know if it is realistic, but if it is, I think there is a solution. When there is a siege and several waves of defenders are crushed, the remaining garrison just vanishes and there is a message that the remaining defenders retreated or something like this. Perhaps, the same mechanism could be used for a usual battle. When morale drops to a low enough level (for example, "enemies flee in terror") retreating untis don't have to reach the border of a map to dissapear. They instead vanish where they are at