If the thumbnails in the shelf appear to be different than habitual it may be because of the shader used to render the previews.
|Broken thumbnails||Normal thumbnails|
1 - Open the main settings window
Go to Edit and click on Settings :
2 - Remove the Shelf preview shader
In the General view scroll down until the "Preview options" section is visible.
Click on the cross button in front of the "Material preview shader" to remove the current shader specified.
3 - Restart Substance Painter
In order to regenerate the thumbnails so that they can look correct Substance Painter needs to be restarted.