To reproduce:
1. Open attached project;
2. Open "Level1" scene;
3. Make sure Single Pass is enabled (Edit > Project Settings > Player > Stereo Rendering Method);
4. Enter Play mode;
5. Look at the scene with Oculus Rift Headset
-Note how dark it is in the Headset compared to the Editor.
6. Disable "Canvas-Left" in the Hierarchy;
7. Repeat step 5.

Expected result: Lighting intensity is the same in the Headset and the Editor when Single Pass is enabled.
Actual result: Scene in the Headset is darker compared to the Editor when Single Pass is enabled.

Reproduced on versions: 5.4.5p1, 5.5.3p1, 5.6.0p1, 2017.1.0b1.

Not reproduced with HTC Vive (tested on version 5.6.0p1).