To reproduce:

Add a RadDock with ToolWindow, AutoHide the window and open it. You will see that the splitter is big (12px width) and has a black border on the top left corner

Workaround:

Subscribe to the DockStateChanged event to hide the problematic element:

void dock_DockStateChanged(object sender, DockWindowEventArgs e)
{
    if (e.DockWindow.DockState == DockState.AutoHide)
    {
        ((e.DockWindow as ToolWindow).TabStripItem.TabPanel.Parent as AutoHideTabStrip).SplitPanelElement.Children[2].Children[0].Visibility = ElementVisibility.Collapsed;
    }
}