If you are editing a Scene containing Prefabs with GameObject icons assigned to them and then open their root Prefab in isolation mode, the GameObject icons for the prefabs in the active Scene will appear in isolation mode.

1. Install Unity
2. Open the attached project
3. Open TestScene
4. Find the TestCube prefab in the Project View and open it in Isolation Mode
5. The icons from the TestCube Prefab instances in TestScene will be drawn in Isolation Mode

Result:
GameObject Icons for the prefab instances in the active scene are drawn.

Expected Result:
Only the GameObject Icon for the prefab being edited should be drawn while in Isolation Mode.

Found in 2018.3.0a6