Sub folders don't stay collapsed.
Folders or any sub folders need to remember the last used state. So when you leave the website and return you have the same view as when you left. All folders shown the way you want them. For example for me I have multiple sub folders that I don't need to see on a regular basis so I want them to stay collapsed but each time I launch the site they are expanded again so I have to manually collapse them again. Annoying.