Reproduction steps:
1. Open the attached "Khaos.zip" project
2. Navigate to the "Adytum" folder ( Assets > Production-Prefabs > Adytum)
3. Observe the thumbnails

Expected results: Thumbnails of the prefabs are visible
Actual result: Empty grey thumbnails are visible

Reproducible with: 2023.2.0b6, 2023.2.5f1
Not reproducible with: 2021.3.34f1, 2022.3.17f1, 2023.2.0b5, 2023.3.0b1

Reproducible on: Windows 11
Not reproducible on: no other environment tested

Notes:
- 2023.3.0a1 was never available and is impossible to test
- Loading the thumbnails would sometimes require reimporting the prefabs
- Possible workaround: Select the prefabs and in the Inspector check the 'Static Shadow Caster' property (Thumbnails remain after unchecking it)
- Changing the icon of the prefab or enabling/disabling the prefab, would temporarily show the thumbnail
- Reproducible on a new project, but you have to create around 10 prefabs for the issue to appear