BagMan64
to
Hey, I got it to work. Thanks for bringing this to our attention. It was a very sneaky problem.
What happened was that Unreal optimized your mesh. In that process, it fused together some vertices along the fracture seams.
Unreal would fuse vertices together if all their vertex attributes are identical in value, i.e. if they have the same position, UVs, colors, normals, tangents... This is typically not a problem, but it became a problem when vertices along your seams happen to have all same attribute values for every attribute.
Because your mesh was perfectly flat, so all the vertices will get the same normal. The UVs you provided had the same values for vertices on either side of the seams. Because the tangents are computed from normals and UVs, they also ended up being the same. So Unreal optimized/fused them. You will noticed the broken version had a fewer vertices in Unreal.
The reason it worked when you did not provide UVs was VAT ROP auto computes UVs for you when they are missing, and the auto-computed UVs moved vertices on either side of the seams into their own separate UV islands. So in Unreal, those seam vertices are not identical in every attribute.
My fix was adding a small 0~0.0001 range random UV offset to the vertices along your seams. You won't notice that small of a UV discrepancy, and it side steps the problem. The result: