Licensee reports an issue with MeshDescription indexing when reversing a degenerate triangle.
Somehow arrange to have a mesh with a degenerate triangle where one of the vertex indices is repeated (this should never happen), instance it with a mirrored scale, and try to merge it with another static mesh.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-183328 in the post.