Apparently the project is software raytracing enabled by default, but enabling hardware raytracing on a 3090 and it suddenly looks like this:
Super easy answer to this one (I’ve encountered it too):
hardware ray-tracing uses proxy geometry by default, not the full meshes themselves, for performance reasons. This manifests as geometry shadowing/reflecting itself, which is your problem here. Lumen reflections and GI have a fix for this by default, which is something like raytracing.shadows.avoidselfintersectiontracedistance 1 (or something like that). It should solve your problem instantly.
Another option (albiet a not great one), is to force nanite geometry to be put into the ray-tracing BVH directly via raytracing.nanite.mode 1. It’ll be more VRAM expensive, but the RT shadows will be actually interacting with the same geo as the Gbuffer, so should work.