Material preview images are missing in the Project window/Asset Selector window.

Steps to repro:
1. Create a new HDRP template project.
2. Project Window > Select Two Column Layout.
3. Select Assets/ExampleAssets/Materials folder.

Actual Result:
Material preview images are missing in the Project window.

Expected Result:
Material preview images are showing in the Project window.

Reproducible in:
2020.2.0a12, 2020.1.0b9, 2020.1.0b6 with Package version: 8.1.0

Working Fine in:
2020.2.0a12, 2020.1.0b9 with Package version: 9.0.0, 8.0.1

Environment:
Occurring on Windows 10 & Mac 10.15.