How to reproduce:
1. Create a new project
2. Create a new Material
3. Observe the Material's Thumbnail
4. Make any changes to the Material
5. Observe the Material's Thumbnail once more

Expected results: The Thumbnail for the material changes according to the property changes and the background remains the same
Actual results: The property changes are reflected in the Thumbnail, however, the background turns darker grey except for the top-right quarter which is being colored in a lighter grey.

Reproducible with: 2022.1.0a3
Not reproducible with: 2019.4.29f1, 2020.3.14f1, 2021.1.16f1, 2021.2.0b4, 2022.1.0a2

Notes:
1. Saving the project or re-importing the material asset returns the background color to the original state
2. The issue is reproducible on macOS 11.4 and Windows10