FBX material IDs from 3DS Max are reordered in automatic LOD generation

• This appears to be a UE4 bug in the way it handles the importing of FBX Material IDs from 3DS Max exports.
• Max uses set Mat IDs to organize multiple materials on a single object.
• UE4 correctly handles this when the object is first imported.
• When generating automatic LODs or re-importing the mesh, the order of mat IDs no longer matches and the Materials on the mesh may appear in the wrong location for the various LOD levels.

Workaround: UE4 appears to handle Mat IDs in the same way Maya does. This is somehow based on the objects position, scale, or bounding box. Organizing ID set in order from largest to smallest in Max before exporting will result in proper order generation in UE4.

Versions tested:
4.19.2 - CL 4033788
4.20.2 - CL 4302132

This appears to be a regression as the issue does not occur in 4.19.

1. Download the attached FBX file, OR create and combine 4 meshes in 3DS Max with 4 separate materials applied then export as FBX
2. Import the FBX into UE 4.20
3. Open detailed static mesh editor, apply any 4 random Materials to the available slots
4. Set LOD Group to Level Architecture to auto-generate LODs
5. Preview various LODs with LOD Picker or by scrolling in and out on the object

Result: Material order on the LOD will change and the materials in the viewer will be on the wrong location of the mesh.
Expected: Materials will appear in their correct location on the mesh.