Hello. When putting Roughness to 0 on a Metallic Material Setting (Using a Metallic map here, so it's set to 0 or 1) for Principled Shader in Blender 2.8 on MACOS 10.14.5 using a Vega 64 AMD as external EGPU the material gets white. Using Blender 2.8 latest built from june and also latest built of RPR. See Pictures below for Reference.
This doesnt happen in Cycles, and therefor can be a bit of a pain when switching between the two. Anybody else experiencing this? Hope i could save someone else some time.
This is the VEGA macOS driver or Metal 2 issue in 10.14.5 (and all betas for 10.14.5).... it is, however, fixed in the 10.14.6b2 onwards and in Catalina.
If you render this with a Polaris AMD GPU (e.g. RX 580) it will render 100% correctly in 10.14.5.
I'm suffering with this issue too. Hopefully macOS 10.14.6 will be out soon.