Materials are broken when merging a mesh with another that has negative scale
Licencee states that bisMirrored in FMeshMergeHelpers::TransformRawMeshVertexData seems to be involved.
Does not occur in 4.17
1. Import or Create a mesh with multiple material channels (attached)
2. Place Mesh
3. Place another mesh, mirror it with Scale(-1,1,1), Scale(1,-1,1), Scale(1,1,-1), or Scale(-1,-1,-1)
4. Merge both actors with default settings
5. Observe strangeness with materials in merged mesh
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-56953 in the post.