Currently in snapshot 1.16.2 Pre-release 1 and the latest Minecraft release 1.16.1, editing NBT motion data while riding either a horse, strider, or boat is inconsistent in being able to run, and the height launched is lower than expected when it does run.

It was expected for the entity with the player riding it to go up smoothly and run the command consistently when motion being edited, but only triggered around 10% of the time, and players riding the entity did not reach full expected height. (Side note: For some reasons, saddled pigs and minecarts off of rails worked perfectly fine with motion editing )

Steps: 

-Ride a strider, boat, or horse

-While riding, run the command: "/data modify entity @e[type=!player,sort=nearest,limit=1] Motion set value [0.0d,1.0d,0.0d]" (You might have to use your arrow keys to spam the command until the entity you are riding is launched.)

-Try the same command while not riding said entity. Observe the differences of consistency of being able to run the command and the height launched.