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:
[Link Removed]

Found in 4.18 CL# 3832480, 4.19 CL# 4033788, 4.20 CL# 4369336, 4.21 CL# 4489857, and 4.22 CL# 4498653

Steps to Reproduce
  1. Open the attached 3DS Max file and export the FBX
  2. Open the editor and import the FBX
  3. In 3DS Max Add an additional material ID slot to its multi/sub object material and set one of the green faces to ID 6
  4. Apply a material to the ID 6
  5. Export the FBX again
  6. Right click on the mesh in the Editor and select reimport

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.

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Won't Fix
Affects Versions4.
CreatedOct 25, 2018
ResolvedAug 18, 2021
UpdatedAug 18, 2021
View Jira Issue