So I just discovered from Xamarin that the way that Grid layouts work changed in XForms 1.2.x. Previously, if you did not specify a column definition for a single column grid, it would behave like MSFT SL/WPF/WinStore apps and it would take up the width of it's parent container. Now, it will default to "Auto", and will only take up the width of it's content. This makes the "Star" width in a child grid useless in that case.
This new "feature" apparently is not a bug, but is here to stay.
It would have been nice at the very least for Xamarin to have announced this decision, rather than just dumping it out there for people to find.