Clear Coat Darkens Transparent Material, Fake Shadows

[rubberchicken]

On this project, I wanted a small amount of roughness to the clear polycarbonate that did not affect the transparency. So I enabled coat of 1.0 with 10% roughness with the same IOR of 1.59 as the dielectric. It took me quite a while to figure out that this was causing the material to render darker than it should. In the images attached, the two cover lenses are the same. Only difference being the one on the left is shown isolated. There's a second issue here where the isolated cover renders even darker again if the rest of the assembly is not behind it AND fake shadows is enabled. Turning fake shadows off it renders about the same as the one on the right. That can't be right. Adding fake shadows still improves the one on the right, regardless of the coat but is still darker than I expect. Are both these issues bugs? Or should I be creating the roughness differently?

[funk]

I tried some tests here but can't reproduce the darkening.

We'd really need a scene to figure this out.