Reinforcements just stand around as the battle continues. Formations that are already actively charging or under command delegation continue to fight but reinforcements when the spawn in just stand there while I'm too busy fighting or commanding to notice them. Some horn or other notification should sound that they are on the battlefield so that I don't lose a battle because they failed to start fighting. They could just start charging at the enemy or copy similar formations.
Maybe this is a bug.
Maybe this is a bug.