Workaround:

        void radDock1_DockWindowAdded(object sender, DockWindowEventArgs e)
        {
            DocumentWindow docWin = e.DockWindow as DocumentWindow;

            if (docWin != null && docWin.Text.Contains(Environment.NewLine))
            {
                docWin.TabStripItem.Padding = new Padding(25, 2, 3, 2);
            }
        }