The current workaround is to handle the RadTabItems' PreviewMouseLeftButtonDown event like so:

         public MainWindow()
        {
            InitializeComponent();
            EventManager.RegisterClassHandler(typeof(RadRibbonTab), PreviewMouseLeftButtonDownEvent, new MouseButtonEventHandler(OnClick), true);
        }

        private void OnClick(object sender, MouseButtonEventArgs e)
        {
            var tab = sender as RadRibbonTab;
            var ribbon = tab.ParentOfType<RadRibbonView>();
            if (ribbon.IsApplicationMenuOpen)
            {
                ribbon.SelectedItem = tab;
            }
        }