To start off, I think it’s best to briefly go over the fixes you’ve attempted in order to help diagnose the problem:
As you can see all faces have been flippen into the right direction, and ive even looked on the small edges of the mesh. I have not found any normal flipped the wrong way.
The bug seems to appear on triangles but im still not sure what the fix of this issue could be. As far as i know is that if i publish it as OBJ it fixes the issue but it would cause CSG errors which im trying to prevent.
From this, we can determine the following:
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This is not caused by mismatched normals. Assuming you’ve checked with backface culling on as you implied in your post, then you should be confident that this is not the problem.
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This is not entirely a rendering issue in studio. Considering how this effects only certain areas of your mesh and works fine in the .obj format, it’s most likely something that goes wrong in the exporting step of the process.
By process of elimination, this is most likely the source of your issue:
N-gons, which are faces with more than four sides. Roblox may struggle with displaying them as the geometry might be deemed “impossible” and fail to load in, leaving just an empty space.
The .obj and .fbx formats have different ways of handling these faces on export if I’m not mistaken, which may be the cause of the discrepancy.
While the example shown may be rather extreme, after exporting as an .fbx and importing it in, it shows the same traits:
So, in other words, the faces (or the surrounding ones) may have an extra few vertices that turn them into n-gons. I would recommend you check your model over like this:
Edit > Select > Select All by Trait > (See image below for settings)
Now, if it detects any n-gons, you can go ahead and start manually editing them. You can select spare vertices and do X > Dissolve Vertices to preserve the rest of the edge/face, or just use the knife tool to add edges where needed until everything is either quads or tris.
Hopefully, if you re-export the model now, everything should be fine!
On the off-chance you’re still having issues though, you could attempt to do Edit > Face > Triangulate Faces either over your whole model or just the afflicted areas and see how that goes.
Hope this could help! Your model looks very nice, by the way!
