The base material is 100% metallic, which means there is no diffuse component (black) and the specular highlight (reflection) is colored.
The coat has a white specular, but also no diffuse (pass through).
Both are 100% smooth so both have about the same size specular highlight.
The light here reflects off the coat, masking the metallic base layer specular. In non-specular highlight areas the base layer black diffuse is shown through the coat layer.
The coat adjusts base roughness and darkens the base layer. This can be seen in the bug repro case if you look closely. The spheres and material are not fully black in the highlight area.
In this example there is only a single point light that lights the spheres and produces a specular reflection. Only that area is colored for a metallic material, rest of the sphere is black.
In a more realistic scene there is also indirect light from the environment. This can be achieved with a skybox.
Open Window->Rendering->Lighting->Environment.
Set a sky box texture.
At the bottom, click Generate Lighting->Bake Reflection Probes.
This will give a more realistic look which I suspect matches closer to the expectations.
In general the PBR material model used in URP does not cover all real world materials. And some parameterizations of the model can produce materials that have no practical real world counterpart.