Hello,
Not sure if that is something i never noticed, or some weird bug. If creating a corona-physical material. The Material preview is not showing the base/Diffuse layer if texture map channel is other than 1 if i use a composit map to mix various texturemaps.
All other maps like glossy or bump show correctly.
I probably didnt noticed, when i used straight the map or with legacy materials.
I'm using Max 2020 and corona 7 hotfix1