The bug

Slow Falling normally negates fall damage, but if there is a lag spike timed around the point of reaching the ground, the player can still take fall damage. It seems to happen if the lag spike lasts long enough that the player falls 4+ blocks during the lag spike and reaches the ground as it ends.

I was able to consistently reproduce the bug by reloading while using a datapack that has a bunch of files and takes a couple seconds to load, but, importantly, nothing that affects Slow Falling.

How to reproduce

1. Give yourself Slow Falling
2. Type

   /tp @s ~ ~10 ~

3. As you reach the ground, trigger a lag spike by typing

   /reload

   →  If timed correctly, you should take fall damage.