I have a weird issue with Cycles in Blender 2.92:
This comes from a larger project but I was able to reduce it down to this small example.
The triangle is copied from the project I had issues with and the plane is new, both use the same material, however they render differently:
As you can see at least in Eevee it produces a usable result but in Cycles the triangle is just black, the plane demonstrates that the material is not broken as it works fine there in both rendering engines, so the issue must be with the triangle mesh for some reason but everything I've checked (normals and such) seemed fine.
Any idea what I'm doing wrong?