This repository shows a bug within Xamarin Forms targeting UWP >= 16299.
Issue: [Bug][UWP] Loading wrong Xamarin.Forms.Core.dll
This is still an issue with Xamarin Forms 4.4 and won't get fixed there: Xamarin Forms 4.4 Service Release 6 known issues
But it has been fixed with the initial release of 4.5 (specifically 4.5.0.282-pre4), now at Service Release 4.