When gamerule "naturalRegeneration" is active and fall damage of any caliber is taken, all the player's health will regenerate.
(Similar to the bug in Super Mario 64 where you can dip into water and recover all your health)

What I though would happen:
The player's health should not regenerate.

What really happened:
The player's health regenerated at the same rate as though the gamerule were inactive.

Steps to Reproduce:
1. Activate the gamerule with cheats or a command block
2. Take any type of damage
3. Take any amount of fall damage
4. Health will regenerate despite the gamerule being active