Potions effect can't be get through the entity.

System.out.println(((CraftTippedArrow)entity).getHandle().h.isEmpty());				System.out.println(((TippedArrow)arrow).hasCustomEffects());

Both always return false in EntityDamageByEntityEvent and ProjectileLaunchEvent (didn't test anywhere else)

However, the effect is correctly applied to the shot entities and we can see the particles around the arrow.