This bug is easy enough to reproduce

In the Fast Root create a cell with a Folder

Click on that folder cell

So you are now in the folder

Hit the back button in the new cell to go back to the Root

You are back in the root

You will see that a new cell will have appeared top left

So before

And after

if you click on this back cell you will find yourself in Bookmarks Menu.

So Fast Dial has created an unwanted back cell which leads back into the Bookmarks Menu

Sometimes removing the newly created (non-back) Folder causes this Back cell to disappear automatically.

At other times it refuses to go and there is no way to remove it that I have yet found.

So a new Profile is necessary.

telega comment
ah, must be some error generating "back" image for bookmarks menu folder

Yes, that sounds like exactly what it is. :-)