Custom context menu in ApplicationMenu.AdditionalContent

Hi, in v17 of Actipro, I was able to add a custom context menu within the additional content. That worked in v17 but starting v18, I'm not seeing the custom context menu anymore and I only see the default context menus of "Show Quick Access Toolbar..." and "Minimize the ribbon". Is there another way of doing this now? Here's my code:

<ribbon:ApplicationMenu>
    <ribbon:Button  Label="New" KeyTipAccessText="N" InputGestureText="Ctrl+N"/>
                  
    <ribbon:ApplicationMenu.AdditionalContent>

        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="Auto"/>
                <RowDefinition Height="Auto"/>
            </Grid.RowDefinitions>

            <TextBlock Grid.Row="0" Text="Files:" Padding="5" FontWeight="Bold"/>

            <ListBox Grid.Row="1" ItemsSource="{Binding InternalList}">
                <!--Context Menu-->
                <ListBox.ContextMenu>
                    <ContextMenu>
                        <MenuItem Header="Remove From List"/>
                        <MenuItem Header="Clear List" />
                    </ContextMenu>
                </ListBox.ContextMenu>
            </ListBox>

        </Grid>

    </ribbon:ApplicationMenu.AdditionalContent>

</ribbon:ApplicationMenu>