Adding a material to an FBX with a Multi/Sub-object material, then reimporting the FBX will not import the new material. The material element will be added to the mesh but the material applied will not be imported.
The issue does not occur if you delete the imported items and then import it again.
Video of the issue:
Found in 4.18 CL# 3832480, 4.19 CL# 4033788, 4.20 CL# 4369336, 4.21 CL# 4489857, and 4.22 CL# 4498653
The material added to the multi/sub-object material will not be imported along with the changes with the mesh. Additionally the menu that appears upon reimporting doesn't seem to give much context.
The material added to the multi/sub-object would be imported.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-65797 in the post.