Steps to reproduce:

1. Import 'model_test.unitypackage'
2. Make sure the asset icon size is set to anything bigger (not the list type)
3. Navigate to the 'RollerBall' prefab in project tab
4. Notice the RollerBall prefab is shown as the blue cube
5. Right-click the prefab, select 'Reimport'
6. Notice the prefab icon changes to the ball icon (see attached gif)

Expected behavior: The prefab should be shown with the model preview icon right after importing the package without a reimport.

Reproduced with: 5.0.4f1, 5.2.3f1, 5.4.0a2