Standard Disclaimer: There are links to non-Microsoft websites. I will keep working with you until it's resolved. Feel free to ask back any questions and let us know how it goes. Test moving Pictures folder back to default by turning off Folder protection for it. If nothing has worked Check also if you have OneDrive Folder Protection for Pictures which moves the User Pictures folder into Onedrive and caused my thumbnails to go away until I moved them back. Go into Settings > System > Storage, choose Free Up Space Now, let it scan, check the box for Thumbnails, then choose Remove button.
I'm Greg, an installation specialist and 10 year Windows MVP here to help you.
