Trouble sharing a StreamImageSource from iOS IPhotoChooser

I have a Xamarin.Forms project which uses the native photo choosers on Android and iOS to load an image from the gallery or the camera and display it in a Forms page.

My implementation uses the DependencyService to get the actual photos, and creates a StreamImageSource which I send asynchronously to my Forms page via the MessagingCenter.

I use that ImageSource to show thumbnails in on a page by creating a new Image() and setting Image.Source to that source.. When those thumbnails are tapped, I show a larger image, by creating a larger Image() and setting the same Image.Source to it.

On Android, I have no problems::

var imgsrc = ImageSource.FromStream(() =>
 {
    MemoryStream ms = new MemoryStream();
        bitmap.Compress(Bitmap.CompressFormat.Png, 100, ms);
        ms.Seek(0L, SeekOrigin.Begin);
        return ms;
 });

 MessagingCenter.Send<Object, ImageSource>(this, Constants.MsgPhotoData, imgsrc);

on iOS I do the following:

UIImage originalImage = e.Info[UIImagePickerController.OriginalImage] as UIImage;
if(originalImage != null) 
{
        ImageSource imgsrc = ImageSource.FromStream( () => 
         {
              return  originalImage .AsPNG().AsStream();
         } );
  }
  MessagingCenter.Send<Object, ImageSource>(this, Constants.MsgPhotoData, imgsrc);

In Forms, in m MessagingCenter.Subscribe delegate I do something like this:

var thumbnailImage= new Image();
thumbnailImage.Source = arg as ImageSource,

Later on, if that thumbnail is tapped, I show a larger image by:

var fullSizeImage = new Image();
fullSizeImage.WidthRequest = fullsizeImageWidth;
fullSizeImage.HeightRequest = fullsizeImageHeight;
fullSizeImage.Source = thumbnailImage.Source;

So on iOS, this only works if I don't dispose 'originalImage', because setting fullSizeImage.Source to thumbnailImage.Source calls the

ImageSource imgsrc = ImageSource.FromStream( () => 
{
    return  originalImage .AsPNG().AsStream();
} );

code again.

This is not what I want to do.

The ImageSource has already read the stream. Why does it try to re-read it every time I access it? Is there a way to cache it?