[UE5.5]Why are all UV triangles separating when creating LODs?

Development Asset Creation
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1

I can’t figure out why this is happening. The source geometry is clean. The UV map only has discontinuous UVs around the edges of UV islands. Everything is normal.

But when I use the simplify tool in the Modeling mode or set the number of LOD’s to generate in the static mesh editor, I get this:

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The mesh looks horrible. You can see all the edges of every single triangle. And in most cases, it refuses to create a Lightmap UV because there are too many pieces.

How can I prevent this from happening?

edit: I’m noticing that my original mesh has 94138 vertices, but when I import it into UE, it has 424078 vertices. So clearly the vertices are being duplicated so that each polygon is separate. Why is it doing this?

2

Looks like it’s doing this because of sharp edges. A vertex can only have one normal. So if there’s a sharp edge, it’ll need more than one normal so it duplicates the point. What I don’t get is why it moves the UV’s around like that though.

3

I’ve imported custom LODs, but now building lighting crashes. Debugger says that LODData array has one element, but it’s trying to process all LODs so I get an index out of bounds.