Sometimes when I fall down and do nothing at all, the fall damage does not register. I made sure the gamerule fallDamage is on, but when I tried it, it still does not register the fall damage. I am not sure with the versions, but this has been going on for a long time now. Thanks.

Steps to recreate or to do the bug:

1 - Build a 20+ block high platform.

2 - Set your Gamemode to Survival or Adventure.

3 - Jump off the platform.

4 - Once you jump off the platform, do not move your camera. Either hold down the space bar or sneak once you get to the ground.

5 - There is a chance that it ignores fall damage.