The crouch_one_cm objectives stat does not work when you jump before sneaking.
How to reproduce :
Create the crouch_one_cm scoreboard with this command :
/scoreboard objectives add crouch_one_cm minecraft.custom:minecraft.crouch_one_cm
Display this one in the sidebar :
/scoreboard objectives setdisplay sidebar crouch_one_cm
When you crouch, the value increases. But if you jump and before you fall back to the ground you crouch, the value doesn't increase even if you move.
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Easier way to reproduce: hold the sprint key, start moving, hold the sneak key, then let go of the sprint key. You'll be sneaking but your sprint stat will be going up, and your sneak score won't.
Can confirm in 20w51a.
Can confirm in 21w03a.
Can confirm in 1.16.5 and 21w08b.
Video attached.
Can confirm in 21w16a.

MC-258533 video attached.