Steps to reproduce:
1. Open user attached project (RenderTextureLights)
2. See the scene view, note that the cube is illuminated with pink light
3. See the game view, notice the lighting is cut off at about half way on the cube
4. In the Camera's inspector, set the viewport rect's X position to different values between 0 and 0.5
5. Notice how the cutoff point for the light changes. At 0.5 the light is not rendered at all.

Expected result: the whole cube should be illuminated with pink light on RenderTexture

Reproduced in: 2017.1.0b5, 5.6.0a1
Not reproduced in: 5.5.3p3
Regression introduced in: 5.6.0a1

Fixed in: 2017.3.0a2
Backported to: 2017.1.0p2