Jono, are you watching a low% run? In those runs, it is required to finish the game with 3 hearts, and the heart containers from bosses must be deleted. In order to delete the heart pieces, there needs to be a specific amount of other things on screen so that the game thinks the heart container was picked up and the pendant/crystal will then drop. In the case of Moldorm, there is no known set-up that will drop the pendant after the heart container has been skipped, and so the room must be re-entered, which is done by falling off of the platform and walking back into the arena.