I had a very similar issue, but with Megascans assemblies. It turned out that, for some reason, they had a negative scale transformation applied to some of the instanced meshes, causing mirroring. To explain, if you apply a negative scale to an actor (e.g., using -1 on the X-axis), it mirrors along that axis. This mirroring can result in inverted normals when using path tracing, at least within the Assemblies. Unfortunately, the Assemblies seemed to ‘forget’ about this operation; in fact, from the transformation tab, the scale still showed as 1,1,1. In my case, it was on the X-axis, so I solved it by manually setting the X transformation back to 1 (typing 1 again) and removing the 180-degree rotation on the X-axis (in my case).
follow this steps:
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open your assembly
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find which meshes is being rendered black (Isolate it by hiding them one by one.
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once you found it, go to the details panel, instances tab. You should probably have a rotation somewhere of almost 180°, probably this is it. In my case it was the X axis.
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type 1 on the corresponding scale axis, and put back to 0 the rotation otherwise the meshes will be upside down.
