Hello there,
I think i may have found a bug, for real !
Here is the situation : i've used a layered material and - for optimization purpose - the materials that are connected to it are rayswitch ones.
Of course, the raySwitch has the correct material in reflex / refrac / direct overrides, but a simplified one (dull color simple mat) in the Gi override.
The objects (and the preview) with layered material assigned are rendered with the direct look of the dull simple one, instead of the one plugged into the "direct override" slot.
See attached picture, the teapot should be shiny grey.
Also i've tried to put a mask (coronaColorMap) in the mask slot, to test : same issue.
Edit : Corona is 3 hotfix 2, tested with 3dsmax 2018.4 and 2019.3