Is this behaving as expected? With Filter Glossy at 1, Multiscatter GGX breaks down completely with roughness values between 0.018 and 0.025. This doesn’t happen for Principled afaik, only Glossy and Anisotropic (for aniso = 0) BSDF.
Also it seems incapable of shader mixing with a regular GGX glossy. For both roughness’es at 0.02 (in the problematic span) it’s contribution is black at 0.999 shader mix.
2.80 beta, 2019-04-13 18:58.