Bug with mirrored geometry normals

Development Asset Creation
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1

We encountered a pretty weird and quite reproducable bug when importing from 3DS-max (or probably any other software) into unreal. If we start off from a geometry, and then mirror it and rotate it by 90 degrees, then edit it again, the normals and tangents get some weird artefacts imported into UE5.
These artefacts aren’t visible in Houdini (not even on the geometry spreadsheet level) and they aren’t detectable either in Max.
Here I attached a file to demonstrate it, as well as a picture of the artefacts it shows in UE5.
I tried changing the relevant import settings to no avail, I also tried running the asset through houdini first, same result.
The same mesh works perfectly in UE4, and the same mesh works when we leave out either the rotation or the mirroring.

image_2022-07-01_203134300
image_2022-07-01_203134300845×867 39.1 KB

Building_014_Element_.FBX (23.0 KB)

I suspect an engine bug but I wanted to ask if you think there’s something I don’t know.

2

I’m importing a purchased 3DS Max+V-Ray file and having the same issue. Its a fighter plane so makes sense that there would be lots of mirrored and flipped parts. Even when I flip normals within UE5 on the individual meshes, it creates all kinds of wierd artifacts within the textures and the way the light interacts with it. Using DataSmith to Export and Import from 3DS Max+V-ray to UE5. Some of the issues are partially resolved when I make the materials two-sided, but since nanite doesn’t support two-sided geometry and I’m relying on nanite for this project, I’m in a pickle hehe.