Why does await never return?

I'm seeing an await that never seems to return. Here's the sample code:

public partial class MainWindow : Window, INotifyPropertyChanged
{
    private string _status;
    private CancellationTokenSource _cancellationTokenSource;

    public MainWindow()
    {
        InitializeComponent();

        _status = "Ready";

        DataContext = this;
    }

    public string Status
    {
        get { return _status; }
        set
        {
            _status = value;
            OnPropertyChanged(nameof(Status));
        }
    }

    private void OnStart(object sender, RoutedEventArgs e)
    {
        Status = "Running...";

        _cancellationTokenSource = new CancellationTokenSource();

        StartProcessing();
    }

    private void OnStop(object sender, RoutedEventArgs e)
    {
        _cancellationTokenSource.Cancel();
    }

    private async void StartProcessing()
    {
        try
        {
            await new Task(() =>
            {
                Thread.Sleep(5000);
            }, _cancellationTokenSource.Token);
        }
        catch (TaskCanceledException e)
        {
            Debug.WriteLine($"Expected: {e.Message}");
        }

        Status = "Done!";
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

What happens is OnStart gets called, it sets status to "Running...", then calls StartProcessing. Five seconds lapse, however I never see the status get set to "Done!"

If I call OnStop, then the task is cancelled and I see the "Done!" status.

I'm guessing I'm creating a task as well as the task created by async/await but it's hanging or deadlocking?

Here's the WPF XAML code:

<Window x:Class="CancellationSample.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"
    mc:Ignorable="d"
    Title="Cancellation Test" Height="350" Width="525">
<DockPanel LastChildFill="True">
    <StackPanel DockPanel.Dock="Top">
        <Button Width="70" Margin="5" Click="OnStart">Start</Button>
        <Button Width="70" Margin="5" Click="OnStop">Stop</Button>
    </StackPanel>
    <StatusBar DockPanel.Dock="Bottom">
        <StatusBarItem>
            <TextBlock Text="{Binding Status}"/>
        </StatusBarItem>
    </StatusBar>
    <Grid></Grid>
</DockPanel>
</Window>