reproducing:

both command blocks contain the command: /data get entity @p Motion

Walk forward holding W
When reaching the string infront of the observer the command gets executed while the players is walking forward

the command didn't detect the player's Motion correctly. The player was clearly moving but his Motion is 0.0 .
The same result is been seen when the player flies using creativemode.
I tried it in a nother world and got the same result.
What I expected to happen is the command output showing a Motion like [-1.5,0.0,0.0]

when jumping or using ice you can see a minimal Motion when letting go of W before hitting the string