Broken assignation of materials when merging meshes

See the following UDN for a good visual explanation of the bug:

https://udn.unrealengine.com/questions/512811/view.html

Seems like this was introduced around CL 5639656.

• Create 3 cubes
• Assign each cube a different material (red, green, blue)
• Select the 3 cubes and merge them to create a new static mesh with 3 sections that each reference one of the material
• Place an instance of this new static mesh (let's call it merged mesh) in your scene
• Select it and create a proxy mesh, notice how the proxy mesh is identical to the merged mesh.
• Now, change the assigned materials on the merged mesh for each section to 2, 1, 0 (or any other combination really)...
• Create a new proxy mesh - notice how the colors assigned to each cubes doesn't match those found in the merged mesh.

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