Certain levels have little lighting in ContentExamples on Mac

Certain levels have really dim lighting in ContentExamples on Mac.

This does not occur in //UE4/Release-4.15 on CL 3416026.

Maps tested so far where the issue is noticeable:
Navmesh
UMG
ParallaxOcclusionMapping
Material_Properties
Lighting
Material_Instances

1. Open ContentExamples on a Mac.
2. Select File > Open Level... > Navmesh

Result: Lighting is very dim in the level.

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