If you return back to the superordinate folder very quickly after opening a folder with pictures in it, all of the higher folders get the image of the last opened folder, instead of a “folder symbol”.
E.g. in the attached picture I opened the folder “Stuff” and then returned back to the root folder very quickly. After that all of the folders get a symbol of pictures of the “Stuff”-folder: All of the pictures you see are from the folder “Stuff”, so it’s no folder preview.
Sorry if I cannot explain very well