Category Thumbnails Disappeared

Having a strange problem with my category thumbnails in the admin area which causes a few issues in the front end.

The category thumbnails have disappeared.

When I edit the category and scroll down to the “upload” area it shows an image there. I have tried deleting it and re-uploading it but it still does not show in the category thumbnail.

So I tried deleting a category and adding it back in. Uploaded an image. Shows in the “upload” area but does not show in the thumbnail.

I then tried creating a new category with a slightly different name. Uploaded an image. The image shows up in the thumbnails. But when I delete the OLD category and rename the new category to the correct name the image disappears again.

Any help would be hugely appreciated. I really do not want to create all new categories with new names and reassign all my products ??