It makes no sense that you can just leave mid-challenge and heal your Pokémon without penalty. As soon as you enter the Gym, the Gym Challenge should begin, and if you leave at any point, the Gym Challenge is cancelled and reset. I understand why this wasn't a thing in the early Gens, but this REALLY should've become a thing by Gens 5/6, and DEFINITELY by Gens 8/9.