Incorrect material element indexes when importing multiple meshes in the same FBX. If you re-import the mesh that has the incorrect material element index it will correct itself.
Seems similar to [Link Removed]
Found in 4.19 CL# 4033788, 4.20 CL# 4302132, 4.21 CL# 4321618
Results:
Box001 will be imported with the correct material element index but Box002 will be incorrect.
Expected:
Both boxes would import with the proper materials in each slot
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-63445 in the post.