The bug
While flying around using WASD in creative mode the stat increases, but not if the player only moved by a small amount.
How to reproduce
Create a scoreboard to track the stat
/scoreboard objectives add Fly minecraft.custom:minecraft.fly_one_cmPut it on the sidebar so you see what's happening
/scoreboard objectives setdisplay sidebar FlyTap any of the WASD keys and notice it being stuck at 0, even with key presses longer than a tap
Move just enough to have the score jump to a number, sometimes it increments by 24, 28 or other similar value. This happens during one tick.
This is actually related to speed, not distance, as can be seen by using the mouse scroll wheel to adjust your flying speed when in spectator mode. There is a minimum flight speed required in order for the statistic to increment, and when flying short distances there is not enough time to accelerate to this minimum speed, hence the statistic does not increase.