The bug

In the /particle command, when the <count> parameter is 0 and the <speed> parameter is greater than 0, the particle will use the delta values as a motion vector. "dust" particles used in this way will not inherit the motion; it seems to only modify the unit direction vector, but not otherwise affect the particle's velocity.

Steps to reproduce

• Run the following command to show the intended behavior of particles with custom motion, and observe it south of your position. The delta values can be used to modify the motion vector, changing the direction and the velocity.

  /particle minecraft:campfire_signal_smoke ~ ~1 ~2 0.1 0 0 1 0 force @a
  

• Run the following command to display the bug, which creates a dust particle, and once again observe it southward. The delta values can still manipulate direction, but not velocity.

  /particle minecraft:dust 1 1 1 2 ~ ~1 ~2 4 0 0 1 0 force @a