I used commands to visualize it:

Repeating Commandblocks:

execute as @n[type=pig,nbt={Motion:[0.0,0.0,0.0]}] run say standing

execute as @n[type=pig,nbt=!{Motion:[0.0,0.0,0.0]}] run say moving

This Bug only effects entities facing into the south and the east towards the positive coords.

So {Motion:[1.0,0.0,0.0]} and {Motion:[0.0,0.0,1.0]} turn into {Motion:[0.0,0.0,0.0]}.