It looked as if the hearts passed right through them.Maybe beacuse the heart can only be reunited with its original body.That doesnt explain why the hearts would be come heartlesses upon touching the ground.
If I recall correctly the lore is that the heartless holding captive the heart must first be destroyed followed by the disappearance of the nobody; with the heart free the body can rejoin it. (Also, if the nobody is destroyed first it's just devoured by the darkness leavingg the heart all alone forever.) Why? It would seem a possible answer could be that the body (the nobody) tries to recreate the heart. Perhaps as the body develops this replacement heart, the original heart (perhaps freed from a heartless) can try to come back but the new replacement heart won't let it takes it place. Just a makeshift explanation from me, take it with a grain of salt. As for the hearts as they touch the ground, for one, The World that Never Was is very close to the realm of darkness so the hearts are easily corrupted by the darkness and the heartless easily reform/retake them.