The bug

The minecraft.custom:minecraft.climb_one_cm statistic will only increment when the player climbs upwards, not downwards or sideways.

Steps to Reproduce:

• Run the following commands:

/scoreboard objectives add climb minecraft.custom:minecraft.climb_one_cm
/scoreboard objectives setdisplay sidebar climb
/scoreboard players set @a climb 0

Climb downwards or sideways whilst on a ladder.
→ Notice how the score does not increase.

Code analysis

Code analysis by TriWonder81 can be found in this comment.