First seen here (includes image): https://gaming.stackexchange.com/q/364299/171580

To reproduce, for example:

• summon multiple armour stands on the same height
• give them a levitation effect
  Result: Some of them jump ahead or stay behind for a short while until everything goes back to normal, then it repeats.
• reload world
• summon more armour stands
• give them levitation again
  Result: All armour stands that were created before reloading the world move normally, but the new ones jump ahead and/or lag behind.

This bug is not specific to the levitation effect, it can also be seen when the armour stands are falling. Other ways of movement are probably affected as well, but are harder to see because of the lower speed. This bug is best observed while spectating one of the entities.