Projectiles summoned with motion values greater than ten have the respective set motion set to zero instead. (Eg. If an arrow is spawned with a motion of 10.5 in the X direction and 5 in the Z direction, the arrow motion will become 0 in the X direction but remain 5 in the Z direction)

Steps to reproduce:

1. Run a command similar to 

summon arrow ~ ~3 ~ {Motion:[x.x,y.y,z.z]} 

replacing at least one value with a value greater than but not equal to 10.0. Set other values equal to 0.0

2. Notice that the arrow falls straight down.

3. Change other values as you like, and notice that anytime the value is greater than 10.0, it sets a motion of 0.0, but any other values work.

I've tested this with arrows, snowballs, eggs, and tridents, and it appears to have the same result regardless of the projectile type.

Note that this is different from bug report MC 139548, as this actually affects the motion of the projectile upon spawning with motion, while the previous report only affects the visual elements of it after modifying the motion after spawning.