https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Shaders/Private/PathTracing/Light/PathTracingLightSampling.ush#L128
For the function SelectLight which pick a lights by uniform samping When UNIFORM_LIGHT_SELECTION is defined or a RIS based on radiance CDF. However, random value (RandSample) always be about radiance CDF regradless of UNIFORM_LIGHT_SELECTION defined or not. And InitLightPickingCdf(...) is nowhere used. Or I understand it wrong? 
It’s should be no influence to user, since nobody will use very slow uniform sampling. I found the bug during learning unreal source code.