The bug

After killing a mob, the smoke particles that should appear when the mob despawns after death don't show.

This seems to happen mostly when killing naturally spawned mobs in a newly created world. This may also happen when reloading world and killing naturally spawned mobs. Also, if you reload the world when the mob death animation plays, the smoke particles will not show.

See Video 2. 19w46a.mp4

How to reproduce

1. Create a new world.
2. Get a sword.
3. Find a naturally spawned mob.
4. Kill it. No smoke particles will appear.
5. Spawn a mob with a spawn egg.
6. Kill it. Smoke particles will appear.