It can be fixed by replacing the half-cover folder mask icon with a transparent icon, but that change has to be made to a system file. The steps and icon file are posted here:
https://www.elevenforum.com/t/windows-folder-icons.24280/post-547834
That's the best I can offer for now. Eventually, I hope to streamline those steps to a single clickable patch program.
It would be much better if Microsoft gave users the options to have full size folder thumbnails. It's silly that we have to keep patching out their questionable design choices.