How to reproduce:
1. Open the attached "repro 1234427" project
2. Open the "3D Template" Scene from Assets folder
3. Select the Two Column Layout from the right-click menu of the Project window
4. Select all the Materials from the Assets folder
5. Check and uncheck the "Emission" box in the Inspector window
6. Select the "GameObject" GameObject from the Hierarchy window and change its X position to 2
7. Save the Scene

Expected results: the Material Preview Icons in the Assets folder don't change and continue to represent the actual Material
Actual results: the Material Preview Icons in the Assets folder change after saving and no longer represent how the Material actually looks

Reproducible with: 2017.4.39f1, 2018.4.21f1, 2019.3.9f1, 2020.1.0b5, 2020.2.0a7

Notes:
-Sometimes 6th and 7th steps need to be repeated, especially if the Library folder has been deleted before launch.
-If the 5th step is done again after all the other steps, the icons get fixed
-This doesn't affect the Inspector window Material preview
-This bug reproduces when any changes are saved in the Scene, not specifically ones made on "GameObject" GameObject
-The 5th step can be any change to the Shader of the Material, e.g. Specular Highlights checkbox, Offset coordinates
-This issue also reproduces with URP Material Shaders
-If URP is being used, the 5th step is not necessary as shown in "WithURP.mp4"
-Reimporting Materials one by one doesn't fix their icons
-Refresh doesn't fix the icons
-Reimporting all Assets fixes the icons
-This isn't reproducible with the One Column layout as there are no preview icons