I think this is a bug because if a hero wins a battle while losing all of their troops (remaining only with summoned creatures), they can then be hired in the tavern, where they will again have their starting army. At the same time, a message appears stating that the hero won but lost all of their army and can be hired in the tavern. Or, they will be considered dead in battle. I have encountered both of these scenarios, and I don't understand what determines which one occurs; it may depend on the version of Heroes. In addition, when attempting to get rid of creatures in a garrison, the game always reminds you that there must be at least one creature remaining in the army. Logically, it seems that a hero should not be able to move without an army.