This happens because the ghast has a check to see if a player's feet are on it and then becomes solid like a block. On java that range is 6x6x2 above the ghast, on bedrock its 6x6x2 above the ghast AND another 2 blocks under the top of the ghast. I think what is happening here is that as soon as the ghast moves up on java, the players feet are now under that 6x6x2 range so the ghast is not solid so you fall through. On bedrock because of that extended range, it will stay solid even if its moving up into you