This occurs in //UE4/Release-4.15, //UE4/Dev-Editor, and //UE4/Main
Does not occur in 4.14.3
Result: Observe that upon reimport, all of the material assignments in the material slots are cleared out
Expected: Material assignments should be maintained on reimport
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-41294 in the post.