I have favorites set up in my folder tree for a variety of folders. Earlier today, i noticed that one of them was expandable, allowing me to see the subfolders in the tree structure. This is not normally the case, and no longer is, i somehow reverted it again.
Sub-folders of favorites are expandable (e.g. folders you create in Preferences -> Favorites List, to organise your favorite folders), but the favorite folders themselves aren't.