Reports

Well, with given code it's not clearly reproducible. I refered the code from sample app provided by 'ModernWPF' and tried to create sample with your code. Can you provide code for NavigationTemplateSelector and how you are adding menu items ?
<Window x:Class="WpfApp.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:ui="http://schemas.modernwpf.com/2019"
    xmlns:md="http://materialdesigninxaml.net/winfx/xaml/themes"
    xmlns:rx="http://reactiveui.net"
    xmlns:local="clr-namespace:WpfApp"
    ui:WindowHelper.UseModernWindowStyle="True"
    ui:ThemeManager.RequestedTheme="Dark"
    Title="WPF Toolbox Example" Height="450" Width="800">
<Window.Resources>
    <!--  Navigation menu item  -->
    <DataTemplate x:Key="NavigationViewMenuItem">
        <ui:NavigationViewItem
            Content="{Binding Title}"
            Icon="{Binding Icon}"
            IsEnabled="{Binding Selectable}"
            MenuItemsSource="{Binding Children}"
            SelectsOnInvoked="{Binding Selectable}"/>
    </DataTemplate>
    <!--  Navigation header item  -->
    <DataTemplate x:Key="NavigationViewHeaderItem">
        <ui:NavigationViewItemHeader Content="{Binding Title}" />
    </DataTemplate>
    <!--  Template selector for navigation items  -->
    <local:NavigationTemplateSelector
        x:Key="NavigationTemplateSelector"
        HeaderTemplate="{StaticResource NavigationViewHeaderItem}"
        ItemTemplate="{StaticResource NavigationViewMenuItem}" />
</Window.Resources>
<Grid UseLayoutRounding="True">
    <ui:NavigationView
        x:Name="NavigationView"
        AlwaysShowHeader="True"
        Grid.Row="0"
        IsPaneOpen="False"
        OpenPaneLength="150"
        IsSettingsVisible="False"
        IsBackButtonVisible="Collapsed"
        IsPaneToggleButtonVisible="true"
        ItemInvoked="NavigationView_ItemInvoked"

        >
        <ui:NavigationView.MenuItems>
            <ui:NavigationViewItem Content="Menu Item1" Tag="SamplePage1" Icon="Play" >
                <ui:NavigationViewItem.MenuItems>
                    <ui:NavigationViewItem Content="Mail" Icon="Mail" ToolTipService.ToolTip="Mail" Tag="SamplePage3"/>
                    <ui:NavigationViewItem Content="Calendar" Icon="Calendar" ToolTipService.ToolTip="Calendar" Tag="SamplePage4"/>
                </ui:NavigationViewItem.MenuItems>
            </ui:NavigationViewItem>
            <ui:NavigationViewItem Content="Menu Item2" Tag="SamplePage2" Icon="Save" />
            <ui:NavigationViewItem Content="Menu Item3" Tag="SamplePage3" Icon="Refresh" />
            <ui:NavigationViewItem Content="Menu Item4" Tag="SamplePage4" Icon="Download" />
        </ui:NavigationView.MenuItems>
        <rx:RoutedViewHost
            x:Name="MainHost"
            Duration="0"
            ToolTip="{x:Null}"
            VerticalContentAlignment="Stretch"
            HorizontalContentAlignment="Stretch"/>
        
    </ui:NavigationView>
</Grid>
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