sockets-for-pcl doesn't multicast as expected

Question

I wanted to experiment with UDP multicasting on WP8.1 and found this library on nuget.
I have created 2 apps, one on WP8.1 and one on desktop, I will provide only C# code as WPF is irrelevant here.
WP8.1:

public sealed partial class MainPage : Page
{
    int port = 11811;
    string ip = "239.123.123.123";
    UdpSocketMulticastClient udp;

    public MainPage()
    {
        this.InitializeComponent();
        this.NavigationCacheMode = NavigationCacheMode.Required;
    }

    async protected override void OnNavigatedTo(NavigationEventArgs e)
    {
        udp = new UdpSocketMulticastClient();
        udp.MessageReceived += (sender, args) =>
        {
            var data = Encoding.UTF8.GetString(args.ByteData, 0, args.ByteData.Length);
            Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () =>
            {
                lblChat.Text += "\n" + data;
            });
        };
        await udp.JoinMulticastGroupAsync(ip, port);
    }

    async private void btnSend_Click(object sender, RoutedEventArgs e)
    {
        var msgBytes = Encoding.UTF8.GetBytes(txtEntry.Text);
        await udp.SendMulticastAsync(msgBytes);
    }
}

Desktop (almost identical):

public partial class MainWindow : Window
{
    int port = 11811;
    string ip = "239.123.123.123";
    UdpSocketMulticastClient udp;

    public MainWindow()
    {
        InitializeComponent();

        udp = new UdpSocketMulticastClient();
        udp.MessageReceived += (sender, args) =>
        {
            var data = Encoding.UTF8.GetString(args.ByteData, 0, args.ByteData.Length);
            lblChat.Text += "\n" + data;
        };
        udp.JoinMulticastGroupAsync(ip, port);
    }

    async private void btnSend_Click(object sender, RoutedEventArgs e)
    {
        var msgBytes = Encoding.UTF8.GetBytes(txtEntry.Text);
        await udp.SendMulticastAsync(msgBytes);
    }
}

Now what is happening (debug on an actual phone):

• messages sent from desktop app don't make it to phone app and vice versa
• messages sent from desktop app are visible in the output on desktop app (as expected)
• messages sent from phone app appear in phone app output 3 times (yes - the MessageReceived event fires 3 times)

And on a WP8.1 emulator (running on the same PC as the desktop app):

• messages are being sent between apps
• messages sent from phone app still appear twice on the phone app output

I was expecting this to be very simple task and as you see the code is minimal. Am I doing something wrong? Or is the library faulty (kind of doubt that)? If anyone used this library before, I would appreciate some help, thanks :)