Operating system: Linux-5.4.0-70-generic-x86_64-with-glibc2.31 64 Bits
Graphics card: GeForce GTX 1060 6GB/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 460.39
Broken: version: 2.93.0 Alpha, branch: master, commit date: 2021-03-31 04:29, hash: rBe7f890aa59ad
Short description of error
Specular values less than 0 causes shader in EEVEE viewport to be black.. The same is not observed in cycles. Also tried blender 2.92 and it doesn't have this issue.
Exact steps for others to reproduce the error
Set Specular to a values less than 0 on the Principled shader in EEVEE 2.93
Note: SSR must be off in the Render Properties.
One reason this is an issue is due to the RGB Curve node interpolating values below 0 when using the default auto handle, as shown in the original report images below.
see node setup below
If I disconnect the specular the texture looks fine
Blender 2.92 doesn't have the same problem