Reimported mesh doesn't remove unused material slot

Only once operation of Reimport could provide correct updated material slots

• In 3dmax create a Teaport model with 3 materials assign to it
• Import it into UE4, see there're 3 material slots in the mesh editor which is expected
• Modify the source, reduce materials, only one material get assigned
• Reimport the mesh, see it still has 3 material slot existing in the mesh editor
• Do reimport operation again in Content browser, the material slot gets showing correct material slots now

See the video for reproduce steps: [Link Removed]