To reproduce:

1. Import the package into the project (bugreport 20180503 particle GPU instancing glitch.unitypackage)
2. Open the "demo scene" scene
3. Enter Play mode
4. Observe the ghost cube at the center of the world
5. Select Particle System game object and in Renderer module disable Enable GPU Instancing
6. Observe that ghost cube is gone

Expected: the ghost cube is not there

Reproduced in 2018.1.0a6, 2018.1.0f2, 2018.2.0b3, 2018.3.0a1
Did not reproduce in 2018.1.0a5
Could not test in earlier versions, the GPU Instancing for particles doesn't exist there