How to reproduce:
1. Open the attached "IN-148209_AssetReimportBug.zip" project
2. At the top of the Editor window, press "Repro -> 1 Create Test Prefab"
3. Observe the created "TestCube" prefab in the Project tab
4. Press "Repro -> 2 Toggle Preview Colour (Red-Blue)"
5. Right click on the previously created "TestCube" and select "Reimport"
6. Observe the "TestCube" prefab thumbnail
Actual result: "TestCube" thumbnail remains in blue color after reimport
Expected result: Reimporting makes the "TestCube" thumbnail update and change to red color
Reproducible with: 6000.0.0f1, 6000.0.80f1, 6000.3.20f1, 6000.5.5f1, 6000.6.0b5, 6000.7.0a3
Reproducible on: Windows 11
Not reproducible on: No other environments tested
Note: Thumbnail color only changes after pressing "Reimport all" in the Project tab, or by deleting the project’s “Library” folder