Material renders differently between GPU and CPU ?

[Headroom]

I am experimenting with the Dichroic coated glass https://www.indigorenderer.com/materials/materials/1071.
With Bidir Path it renders as shown in the image in the material library, but when I switch this to Patg/GPU it renders clear.

I am assuming that there is some feature that ins;lt supported on GPU. Just not sure what that is. Is there a document, or post that explains what features are not yet supported on GPU?