A couple of things to note: 1. After being Untrasnfigured, Draco's face is pink and his hair is messy. 2. After being Untransfigured, Draco is still experiencing the pain of being bounced around. What does this tell us? It tells us that Untransfiguration is not turning back time/reverting an object to a previous state. You are not turning an object into what it used to be, you're turning an object into what it used to be + all its subsequent experiences. There is a certain level of object permanence/continuity which cannot be avoided.