What I expected:
Using the particle command with an entity selector would spawn the particles relative to the entities.
What happened:
The particles spawned relative to the the executor.
To reproduce:
Run a command which selects entities such as:/particle heart ~ ~1 ~ 0 0 0 0 1 force @e[type=Chicken]
Observe as the particles spawn around you/the command block.
Use /execute. MC-62638