The bug

stat.fallOneCm does not increase if you drop 0.609552546740035 blocks or below. However, if you drop at higher heights than 0.609552546740035, the score increases.

Note: It appears that it increases even if you don't take damage, as long as you fall over 0.609552546740035 blocks. Can be tested with jump boost. So there is no relation to damage taken to increase fall stat.

How to reproduce

1. Go into Survival mode
2. Create stat.fall_one_cm scoreboard objective

   /scoreboard objectives add fallOneCm minecraft.custom:minecraft.fall_one_cm
   

3. Have the objective be displayed on the sidebar

   /scoreboard objectives setdisplay sidebar fallOneCm
   

4. Be in survival and teleport 0.609552546740035 blocks up and drop to see score does not update

   /tp ~ ~0.609552546740035 ~
   

5. Teleport 0.609552546740036 or more blocks up and drop to see score updates
6. Repeat test with a high jump boost level so that you fall more than 2.1 blocks and notice that the score increments