Yeah, I have the same problem. I rarely use it so its not a huge deal though. There is a workaround. Drag the tab and create a third window. Make sure the window you want to drag to is just below the 'top' window. If you only have a single tab in your 'top' window, when you drag it the window will disappear and the window below it will be visible. You can then drag that tab into the new 'top' window. Its clunky as hell but works. I'm not sure why it does this.
