<telerik:RadWindow x:Class="WpfApp84.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp84" xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"
        mc:Ignorable="d"  Height="450" Width="800" Style="{DynamicResource RadWindowStyle}">
    <telerik:RadWindow.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="/Telerik.Windows.Controls.Navigation;component/Themes/GenericWindows11.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </telerik:RadWindow.Resources>
</telerik:RadWindow>

<telerik:RadWindow x:Class="Example.MainWindow" 
                xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
                xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation" 
                xmlns:helpers="clr-namespace:Telerik.Windows.Controls.Theming.Helpers;assembly=Telerik.Windows.Controls" 
                helpers:WindowEffectsHelper.BackdropMaterial="None" 
                Header="RadWindow"> 

<telerik:RadWindow  xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation" x:Class="Example.MainWindow"
                    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
                    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
                    xmlns:local="clr-namespace:WindowWindows11ThemeTest"
                    mc:Ignorable="d"
                    
                    Style="{DynamicResource RadWindowStyle}"
                    
                    Header="MainWindow" Height="450" Width="800">

private bool isDarkPalette;

private void RadButton_Click(object sender, RoutedEventArgs e)
{
    if (this.isDarkPalette)
    {
        Windows11Palette.LoadPreset(Windows11Palette.ColorVariation.System);
        this.isDarkPalette = false;
    }
    else
    {
        Windows11Palette.LoadPreset(Windows11Palette.ColorVariation.Dark);
        this.isDarkPalette = true;
    }

    this.MergeDictionaries();
}

private void MergeDictionaries()
{
    Application.Current.Resources.MergedDictionaries.Clear();
    Application.Current.Resources.MergedDictionaries.Add(new ResourceDictionary()
    {
        Source = new Uri("/Telerik.Windows.Themes.Windows11;component/Themes/System.Windows.xaml", UriKind.RelativeOrAbsolute)
    });
    Application.Current.Resources.MergedDictionaries.Add(new ResourceDictionary()
    {
        Source = new Uri("/Telerik.Windows.Themes.Windows11;component/Themes/Telerik.Windows.Controls.xaml", UriKind.RelativeOrAbsolute)
    });
    Application.Current.Resources.MergedDictionaries.Add(new ResourceDictionary()
    {
        Source = new Uri("/Telerik.Windows.Themes.Windows11;component/Themes/Telerik.Windows.Controls.Navigation.xaml", UriKind.RelativeOrAbsolute)
    });
}

Tested with 16.8.4 - works as expected. Updated to 16.9.0 to match the client's setup and managed to reproduce the issue. The issue is reproducible also with the 16.10.0 version of Visual Studio.

public MainWindow() // where MainWindow is RadWindow
{
	InitializeComponent();
	SystemEvents.DisplaySettingsChanged += SystemEvents_DisplaySettingsChanged;
}

private void SystemEvents_DisplaySettingsChanged(object sender, EventArgs e)
{
	if (this.WindowState == WindowState.Maximized)
	{
		this.WindowState = WindowState.Normal;
		this.WindowState = WindowState.Maximized;
	}
}

<dpiAware xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">true/PM</dpiAware>

System.EntryPointNotFoundException: Unable to find an entry point named 'GetDpiForSystem' in DLL 'User32.dll'.

   at Telerik.Windows.Controls.InternalWindow.Standard.NativeMethods.GetDpiForSystem()

   at Telerik.Windows.Controls.InternalWindow.Standard.DpiHelper.GetSystemScaleFactor()

   at Telerik.Windows.Controls.InternalWindow.Standard.DpiHelper.CalculateScaleFactor(IntPtr hwnd)

   at Telerik.Windows.Controls.InternalWindow.Standard.DpiHelper.GetPerMonitorDPIAwareScaleFactor(IntPtr hwnd)

   at Telerik.Windows.Controls.InternalWindow.ChromelessWindowHelper.<Initialize>b__81_0(Object sender, EventArgs args)

   at System.Windows.Window.OnSourceInitialized(EventArgs e)

   at System.Windows.Window.CreateSourceWindow(Boolean duringShow)

   at System.Windows.Window.CreateSourceWindowDuringShow()

   at System.Windows.Window.SafeCreateWindowDuringShow()

   at System.Windows.Window.ShowHelper(Object booleanBox)

   at System.Windows.Window.Show()

   at Telerik.Windows.Controls.InternalWindow.WindowWithNoChromeWindowHost.Open(Boolean isModal)

   at Telerik.Windows.Controls.WindowBase.ShowWindow(Boolean isModal)

   at Telerik.Windows.Controls.RadWindow.Show()

   at TelerikWpfCrash.App.OnStartup(StartupEventArgs e) in C:\Users\fedarovich\source\repos\Test\TelerikWpfCrash\TelerikWpfCrash\App.xaml.cs:line 20