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

Steps to Reproduce

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

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ComponentAnim - HLOD
Affects Versions4.184.19
Target Fix4.20
Fix Commit4051828
Main Commit4062611
Release Commit4095966
CreatedMar 27, 2018
ResolvedMay 4, 2018
UpdatedMay 31, 2018