When summoning a llama spit entity with motion, for example using /summon minecraft:llama_spit ~ ~ ~ {Motion:[1.0d,0d,0d]} the motion works fine, with the spit moving smoothly. However if you now were to change the motion using the data command by running /data modify entity @e[type=minecraft:llama_spit,sort=nearest,limit=1] Motion set value [0d,1.0d,0d] it will start to sutter, and only move every few ticks. It also appears to be able to move through other entities. However when relogging it works fine again and the motion is smooth.