The problem is that WindowsFormsHost doesn't work in Popups with AllowsTransparency = True and by default we set it to true in code behind. What you can do is handle the loaded event and change it manually:
sender, RoutedEventArgs e)
var dropDownButton = sender
var popup = dropDownButton.ChildrenOfType<Popup>().FirstOrDefault();
I hope I was able to help you.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.