On survival, the sound that plays together with the damage sound when a player falls from a big height is sometimes played even if the player uses the water bucket to nullify the fall. What I have been suspecting is that the game is allowing a few extra ticks for the player to land on a block and then put a water block on top of it and still not take damage. 

footage of the bug: https://youtu.be/ZVfG2uTYHMk