WebView updating HTML not working for IOS and WinPhone

Hi all,

Since I've updated to XF 1.3 I'm facing more troubles with the WebView. There were already some bugs with the databinding, so I had a work-around for that, which is discussed here. But now I'm facing a problem where the webview is not updated at all for IOS and Windows Phone, but does for Android.

A simple reproduction for this problem:

WebViewPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="FormsWebViewTest.WebViewPage">
  <WebView x:Name="webview" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand"/>
</ContentPage>

WebViewPage.xaml.cs

public partial class WebViewPage : ContentPage
{
        public WebViewPage()
        {
                InitializeComponent ();
                // This works for all platforms, but only one time.
                webview.Source = new HtmlWebViewSource() { Html = "test" };
        }
}

WebViewPage.xaml.cs - OnAppearing

protected override void OnAppearing()
{
        base.OnAppearing();
        // Update webview after x seconds, works for Android, but nothing changes for IOS and WP.
        Task.Run(async () =>
        {
                await Task.Delay(3000);
                webview.Source = new HtmlWebViewSource() { Html = "updated" };
        });
}

Is this a (known) bug? Does anybody has a workaround, so that I can update the HTML in a webview?