When exporting a Static Mesh from within UE4 that has multiple materials, some of the material IDs will be condensed into a single ID which will mess up how the material assignment for that part of the mesh.

An example of this can be seen with the SM_Window from SunTemple that when exported will combine Elements 0 and 3 into a single element on reimport. (This asset has been attached here from 4.10 version, just place folder in Project Folder > Content)

Tested this in:
4.10.4 CL-2872498
4.11.1 CL-2934540
4.12.0 CL-2944947

Steps to Reproduce

1. Download attached asset to project folder
2. Export the static mesh as FBX from UE4
3. Reimport the static mesh and overwrite or import to a new folder in the project
4. Assign the materials to the correct slots using the original as a reference

Regression: No

Results: Material IDs are condensed in some cases resulting in material UVs to not assign the material to the correct faces.

Expected: All material ID assignments should export and reimport correclty.

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Affects Versions4.104.114.12
Target Fix4.13
Fix Commit2950663
Main Commit2973886
CreatedApr 18, 2016
ResolvedApr 20, 2016
UpdatedFeb 4, 2019