How do I prevent the XPSDocumentWriter from rasterizing my VisualBrush?

Question

I use the following code to create an XPS document from a FixedDocument.

using (XpsDocument doc = new XpsDocument(package,CompressionOption.NotCompressed))
{
  XpsDocumentWriter xpsWriter = XpsDocument.CreateXpsDocumentWriter(doc);
  xpsWriter.Write(fixedDocument);
  doc.Close();
}

It works well except in one case where I have a filled path that is filled with a tiled pattern it occasionally rasterizes it and creates an ImageBrush instead of passing along the the VisualBrush. Sadly the image it generates is a terrible representation of the VisualBrush. Its resolution is horrible, and more maddeningly, its does not even fill the original path!

I've done a ton of digging but haven't been able to find a setting that prevents the rasterization.

Any suggestions?