I found this out by accident last evening. Since the update released Tuesday, I can now close folders on my start screen using the back key. I was not able to do this before.
Closing folders using the back key was available even before Tuesdays update. Looks like the problem persisted only on your phone.
Doesn't work for me :/ it just takes me back to the previous app running instead. Running the latest update too
How about without any apps running?