The bug

It appears that /particle spawns particles in all dimensions, even when executing 'at' an entity, or using the 'in [dimension]' argument.

How to reproduce

1. Put in a repeating command block: execute in overworld run particle flame ~ ~1 ~ Make sure it is in the spawn chunks so it stays loaded.
2. Run the command: execute in the_nether run tp @s to get to the nether.
3. Notice the particles are appearing where they are being spawned in the overworld.