By default VRayFastSSS2 uses a simple approximation to single scattering which is similar to diffuse lighting and is computed in a similar way - which is why it goes into the VRayLighting render element. It generally makes the material appear slightly more realistic. If you set the single scattering to “none”, it will go away (see attachments). In previous versions of V-Ray, we attempted to move this contribution to the VRaySSS2 render element and subtract it from the lighting, but it tended to produce negative results sometimes and made more problems than it solved. This is why it was moved back to where it was originally - to the VRayLighting render element. Adding this simple single scattering to the VRayLighting element removes the negative results and makes compositing with the light select elements more predictable.
If you don’t need this single scattering approximation, you can set it to “none”.