Player y motion is reset when moving on ground

The players y motion is set to 0 every 20 ticks and whenever moving when on ground.

Steps to reproduce:

Output the players y motion using a repeating command block: '/tellraw @p {"nbt":"Motion[1]", "entity":"@p"}'

Stand still then move the camera around, stand still again then move the player around.

Expected:

y motion is a constant value: -0.0784.

Actual:

y motion is set to 0 every 20 ticks or whenever the player moves.