XF 4: CollectionView GridItemsLayout with bindable Span property

When I try and bind the GridItemsLayout Span property to the ViewModel, it doesn't work.
See sample project (open CollectionViewPage from landing page):
https://github.com/InquisitorJax/Xamarin-Forms-4-Layouts

code
https://github.com/InquisitorJax/Xamarin-Forms-4-Layouts/blob/master/XF4Layouts/CollectionViewPage.xaml

Is this by design - or a bug?