Repro:

Download and import the repro project here: https://drive.google.com/file/d/1JeBvhAx6SYw3f7xcfBR9IJJ7eJEWgULT/view?usp=sharing

- Open the Scene SampleScene.unity
- Observe the corruption in the Scene view, and the warnings in the console
- Select the asset FR_SectionA_01.fbx
- In the model's inspector, disable the option "Keep Quads" and apply

Observed:

When Keep Quads is disabled and the resulting mesh is triangulated, the pathtracer behaves as expected. It doesn't seem to like quads or ngons.

Expected:

Quads should be considered very likely and Unity does support this as an import option. The pathtracer should be able to handle meshes with non-triangle geometry. Or if it cannot, a meaningful warning should be displayed in the console to advise users how to fix the corruption.