this bug happens on 2020.3 and 2021.1,
in 2021.2 and 2022.1 it is fixed.

Reimporting Prefab or Prefab +Material doesn't update the preview, also restarting Editor doesn't update it. The only thing that updates preview to proper Material color is Reimport All.

How can we reproduce it
1 Create new 3D project
2 In scene create Cube and drag and drop it to Project Browser to get a Prefab
3 In Project Browser create new Material
4 Assign created Material to the Cube Prefab
5 Change Material color
6 Save by pressing Ctrl+S and observe Prefab's preview
7 Change Material color again
8 Save by pressing Ctrl+S and observe Prefab's preview

Actual results: on Step 8 and sometimes on Step 6 the Prefab's preview is not updated with Material color

Expected results: both on Step 8 and Step 6 the Prefab's preview should be updated with Material color

Reproduces on: 2020.2.0f1, 2020.3.0f1, 2020.3.19f1, 2020.3.20f1, 2021.1.21f1, 2021.1.25f1, 2021.2.0a4,
Not reproduces: 2021.2.0a5, 2021.2.0a7, 2021.2.0a9, 2021.2.0b2, 2022.1.0a12

so it was fixed by some change that landed in 2021.2.0a5,
and since 2020.3 is LTS we should backport that fix