The bug is not that they're not affected by gravity. They're supposed to do that, because they always float on top of a water block if there is one. The bug is that they don't check if the water they're in is moving. If it moves up, they are forced to move up as well by the water surface, obviously, but when it moves down, they stay where they are. Kinda like what would happen in space: If something pushes you up, you go up with it, but if it goes down again, you stay where you are.
I'm not entirely sure, but I think the same thing can happen to Jet Jaws.