Fluent Ribbon SelectedTabItem or SelectedTabIndex

I'm using Fluent Ribbon in a MVVM design pattern, with a View/ViewModel. My goal is to change the body according to which tab is selected. I could use either SelectedTabItem or SelectedTabIndex and lookup the corresponding view. However, neither fire when the tab selection changes. In the View I have...

<Fluent:Ribbon Grid.Row="0" SelectedTabItem="{Binding SelectedRibbonTab}" SelectedTabIndex="{Binding SelectedRibbonTabIndex}">
</Fluent:Ribbon>
<ContentControl Grid.Row="1" Content="{Binding RibbonTabContent}"/>

In the ViewModel I have...

    // Used both Item and Index for troubleshoothing, but need only one or the other for implementation

    private IDictionary<string, FrameworkElement> RibbonTabViews;
    private List<FrameworkElement> RibbonTabViewsList;

    public RibbonTabItem SelectedRibbonTab
    {
        get 
        { 
            return selectedRibbonTab;
        }
        set
        {
            Update(() => SelectedRibbonTab, ref selectedRibbonTab, value, false);
            UpdateContentControl();
        }
    }

    public int SelectedRibbonTabIndex
    {
        get
        {
            return selectedRibbonTabIndex;
        }
        set
        {
            Update(() => SelectedRibbonTabIndex, ref selectedRibbonTabIndex, value, false);
            UpdateContentControl(value);
        }
    }

    public FrameworkElement RibbonTabContent
    {
       get { return ribbonTabContent; }
       set { Update(() => RibbonTabContent, ref ribbonTabContent, value, false); }
    }

    protected void UpdateContentControl()
    {
        RibbonTabContent = RibbonTabViews[SelectedRibbonTab.Header.ToString()];
    }

    protected void UpdateContentControl(int index)
    {
        RibbonTabContent = RibbonTabViewsList[index];
    }

I know I don't need both bindings but for the sake of troubleshoot I have both Item and Index. I would think in the ViewModel SelectedRibbonTab and SelectedRibbonTabIndex would be called each time the tab changed. Unfortunately, that doesn't appear to be the case. I have breakpoints at the setters and getters for each and neither are hit when changing tabs. What am I missing? I've been using this approach for years with Microsoft Ribbon but for some reason these don't fire in Fluent Ribbon. Any help would be great, thank you.

You have to set the binding mode to TwoWay to get updated values in your ViewModel.