Items of invisble tab are shown

[pschimmel]

In my application I'd like to make some ribbon tabs invisible if no project is loaded.
However if I set the Visibility property of the first RibbonTabItem to Collapsed the content of the tab item like RibbonGroupBoxes and Buttons are still shown.
I reduced my sample to the following XAML to be able to reproduce the issue. No code behind needed.

<Grid x:Name="mainGrid">
  <fluent:Ribbon>
    <fluent:RibbonTabItem Header="Start" Visibility="Collapsed">
      <fluent:RibbonGroupBox Name="Calculate" Header="Calculate" Icon="pack://application:,,,/NAS;component/Images/Calculate.png">
        <fluent:Button LargeIcon="pack://application:,,,/NAS;component/Images/Calculate.png" Icon="pack://application:,,,/NAS;component/Images/Calculate.png" Header="Calculate"/>
      </fluent:RibbonGroupBox>
    </fluent:RibbonTabItem>
    <fluent:RibbonTabItem Header="Help">
      <fluent:RibbonGroupBox Name="Help" Header="Help" Icon="pack://application:,,,/NAS;component/Images/Help.png">
        <fluent:ToggleButton LargeIcon="pack://application:,,,/NAS;component/Images/InstantHelp.png" Icon="pack://application:,,,/NAS;component/Images/InstantHelp.png" Header="Instant Help"/>
        <fluent:Button LargeIcon="pack://application:,,,/NAS;component/Images/Website.png" Icon="pack://application:,,,/NAS;component/Images/Website.png" Header="Website"/>
      </fluent:RibbonGroupBox>
    </fluent:RibbonTabItem>
  </fluent:Ribbon>
</Grid>

BTW: I'm using version 3.6.1.236