This issue can be seen even in the sample project that comes with the XAM140 class materials which makes me think if it doesn't work in a simple example that comes direct from Xamarin then it isn't something I set up wrong.
XAML pages in the UWP project or iOS project open in Visual Studio's XAML editor just fine. TipCalculator.UWP|MainPage.xaml for example
XAML pages in the portable project open in the XML editor not the XAML editor. StandardTipPage.xaml for example
This is more than _just _missing intellisense, though that's a big problem too. By opening in the XML editor there is no XAML specific behavior to the editor such as automatic tag matching & updating, no F12 code to source, XAML editor plug-ins for cleaning up aren't available
Attached JPG helps show the problem.