Steps to reproduce:
1. Download and open the attached project
2. Disable the "bg" culling mask on the "Directional light"

Expected result: lighting should stay roughly the same, "Directional light bg" lights up that layer
Actual result: no lighting on the background

Reproduced on 5.6.0b1 5.6.0f3, 5.6.1f1, 2017.1.0b6
Not reproduced on 5.5.2p3

Notes: switching to 3d scene view "fixes" the problem, having the camera set to orthoraphic reproduces the problem there