So since I upgraded my Blender installation from 2.92 to 2.93 I encountered this problem with every shader where the roughness is set to zero: Shader gets completely black!
As you can see in the next GIF, I set the roughness to absolute zero and no matter the other settings (specular & metallic) shader always stays black. I then switch to Cycles and, as expected, works properly.
Curious thing is… this problem starts to occur before reaching zero; it appears to be that the threshold is 0.003726621. At 0.003726622 or above, shader works right. At 0.003726621 or below, shader gets completely black.
This is a devastating hit on backwards compatibility due to me having many shaders with zero roughness that are now completely broken.
FYI, this problem appears only on Blender 2.93 and 2.93.1 (latest version), on Linux as well as on Windows. Blender version 2.92 deals with zero-roughness shaders just as well as expected.
Thanks in advance and I really hope you can help me resolve this.