The bug

As mentioned above the Hit Ground frequency does not activate if the player is moving when they hit the ground. Instead it activates the Walk frequency.

Steps to Reproduce

1. Place a sculk sensor at ground level that outputs it's signal into a command block with the command below (note: "x y z" must be set as the sculk sensor's coordinates):

   tellraw @a {"nbt":"last_vibration_frequency","block":"x y z"}

2. Build a platform 11 (or more) blocks above the sculk sensor to avoid detection of anything other than the player falling.
3. Pay attention to the outputs of the sensor in the following two scenarios. Walking off of the platform while not moving in any direction until you hit the ground, and, in the second scenario, walking off the platform, and continuing to move forward until you hit the ground.

An additional command block was placed behind this one to give the player instant health after falling. This last command block is not required for the setup, but recommended for prolonged testing.

Observed Behavior

The redstone signal output of the sculk sensor will be different for the two falling circumstances.

Expected Behavior

The output would remain a consistent number (5 in this case) regardless of how the player fell.

Code analysis

Code analysis by pr0cess can be found in this comment.