Import the attach skeletal mesh, set the second material slot to use the first material then export the asset.
Open the exported asset in a DCC and verify if the materials are set properly.
Result there will be a material name Fbx_Default_Material_1 for the second material
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-205483 in the post.
|UE - Editor - Content Pipeline - Import and Export
|Jan 31, 2024
|Feb 23, 2024