In intern/cycles/blender/blender_object.cpp, in line 529 there is a typo:

In D3607#150148, @Brecht Van Lommel (brecht) wrote:

@Juan Gea (juang3d), this is how things work in Renderman and Arnold (and for what it's worth, I'm the one who implemented LPEs and light groups in Arnold).. The group name is a property of the light there. It also fits better with LPEs if every light only has a single group, and will have better performance.
Even if lights are only in a single group, it's still possible to sum light groups together, but if it takes more setup time then. In my opinion we should make prioritize the typical use case.
@Alex Fuller (mistaed), I think your fix indeed works, but it might be obsolete once I've made further code changes here, not sure yet.