The following sample shows how to load tile packages with the ArcGIS Runtime SDK for .NET Xamarin Forms UWP: https://developers.arcgis.com/net/forms/sample-code/create-terrain-from-local-tile-package/
Refer to the Xamarin Forms documentation regarding how to access specific file paths. Please contact Technical Support if the issue continues to affect the project in a supported release of the ArcGIS Runtime SDK for .NET, please contact Technical Support.