Looks like there’s one sneaky thing going on inside GPULightmass. GPULightmass uses ‘Two Sided Emulation’ by default on all two sided materials - which means it tries to make all surfaces emit on both sides, each side with half energy, which could be the problem why people are seeing darken environments and light leaks. The original purpose of this feature(bug?) was to approximate the behavior of scattering of things like leaves in a forest so that you get ‘green-ish’ environment lighting atmosphere, or papers and thin curtains.
For the skylight multibounce, since GPULightmass doesn’t uses photon mapping the skylight gets natural support of multiple bounces.
Currently the workaround is to turn off two sided on these materials. While it should be a not so hard fix on my side, I’m scheduling my relocation so I cannot write codes in recent days. Sorry for that!