wierd menuitem issue: whole menu closes once deeper than 2 menuitems

<ui:MenuItem x:Name="StartMenuTab" SubmenuOpened="ListStartMenuItems" Header="Start Folder Apps" Icon="{ui:SymbolIcon AppsList20}">
    <Separator/>
</ui:MenuItem>

private void ListStartMenuItems(object sender, RoutedEventArgs e)
{
    StartMenuTab.Items.Clear();

    string[] placeholderDirs = { "test","test","test","test" };
    foreach (var subdir in placeholderDirs)
    {
        var subFolderMenuItem = new MenuItem
        {
            Header = subdir,
            Icon = new SymbolIcon(SymbolRegular.Folder24),
        };

        var placeholder = new MenuItem
        {
            Header = "Placeholder",
        };
        subFolderMenuItem.Items.Add(placeholder);

        StartMenuTab.Items.Add(subFolderMenuItem);
    }
}