To reproduce:
1. Open attached project;
2. Open "Scene" scene;
3. Enable VR (Edit > Project Settings > XR Settings);
4. Build & Run the project;
5. Look into the scene via Oculus Rift headset;
6. Disable VR and repeat 4-5 steps.

Actual result: the view in the headset is darker than the view when VR is disabled.
Expected result: the view in the headset is as light as in the game view when VR is disabled.

Note: it does not reproduce in the Editor.

Reproduced on versions: 2017.2.0b1, 2017.2.1p4, 2017.3.1p1, 2018.1.0b8.
Not reproduced on versions: 2017.1.3p1, 2017.2.0a4.

Tested and reproduced with Oculus Rift DK2