I think I've noticed the same behavior.
With a windows file explorer window open, if you middle-mouse click on a folder or drive in the primary view port, it will open a new tab. However if you middle-mouse click on any shortcut in the left pane of the window, where the drives, file tree, and quick access list is, those will not open a new tab.
It also sounds like he wants tabs to open in the background when you middle click instead of opening the tab and having the new tab take focus.