While I was playing a game with my friends, we decided to grief the world with some lava and water at the top of the build limit. We later found out that if you hopped into the water, the gravity kinda decreases.
Video demonstrating the issue: https://www.youtube.com/watch?v=S7C-6L-GYV4
Place water at the very build height, then, hop into it sinking all the way down to the block. Hop out and you should only have lower gravity at 256 and above. Anything lower will cancel this movement.