How to reproduce:
1. Open user's attached "FBXImport.zip" project
2. Open "SampleScene" Scene
3. In the Hierarchy window expand "MultiMaterial_3dsmax_3" and select "StandardMaterial"
4. In the Inspector window expand the Material tab
5. Go to the Project window and in it select "MultiMaterial_3dsmax_3" (mouse left-click)
6. Reimport the selection (mouse right-click > Reimport)

Expected result: Shaders in the Scene view get updated ("StandardMaterial" spheres attain their shine)
Actual result: Shaders in the Scene view do not get updated

Reproducible with: 2019.4.8f1, 2020.1.1f1, 2020.2.0a21
Could not test with: 2018.4.26f1 (removing nonexistent dependencies from "manifest.json" lead to code errors such as "Assets\MaterialDescription.cs(8,49): error CS0246: The type or namespace name 'MaterialDescription' could not be found (are you missing a using directive or an assembly reference?)")

Notes:
- Step 4 updates the Shaders of the "StandardMaterial" in the Scene view
- Shaders can be forced to update after step 5 by selecting the "StandardMaterial" in the Hierarchy window
- "OnPreprocessMaterialDescription" is used in "MaterialDescription.cs" (this class derives from "AssetPostprocessor")