ProxyLOD::FQuadricMeshSimplifier::OutputMesh may remove duped verts.
The input vertex buffer was resized by numVerts from FQuadricMeshSimplifier before calling OutputMesh.
So these behaviors generate uninitialized vertices at the end of the vertex buffer.
This vertices may be accessed by a process that iterates over the vertex buffer not through the index buffer (e.g., UVAtlus).
Here's a workaround.
Couldn't find a way to repro, may depend on input and simplification settings
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-99466 in the post.