WindowsStateChanged event does not raise

Hello,

I'm in the process of migrating a wpf application from Actipro 15.1 to 16.1

Everything's ok except it seems the new WindowsStateChanged docksite event never raises. 

private void RegisterEventHandlers()
{
_dockSite.WindowsClosed += DockSiteOnWindowsClosed;
_dockSite.WindowsClosing += DockSiteOnWindowsClosing;
_dockSite.WindowsStateChanged += DockSiteOnWindowsStateChanged;
_dockSite.WindowActivated += DockSiteOnWindowActivated;
}

 The 3 others events (Closed, Closing, Activated) raise properly, but I can't make the WindowsStateChanged event to raise (It should raise when I dock/undock manually any toolwindow right?)

private void DockSiteOnWindowsStateChanged(object sender, DockingWindowsEventArgs e)
{
   //Dostuff
}

 My docksite:

        <docking:DockSite Name="PART_DockSite" 
                          Style="{StaticResource DockSiteStyle}"
                          ToolItemContainerStyle="{StaticResource ToolWindowStyle}"
                          ToolItemsSource="{Binding ToolItems}"
                          ToolItemTemplateSelector="{StaticResource ToolItemTemplateSelector}">
            <docking:Workspace>
                    <docking:TabbedMdiHost>
                        <docking:TabbedMdiContainer x:Name="documentContainer"/>
                    </docking:TabbedMdiHost>
                </docking:Workspace>
        </docking:DockSite>

Any idea? Thanks!