Hi Mark,
This may be a bug that we are currently working on. Try the following workaround and see if is helps:
Before you set the MapView.Map property to contain the .MMPK's map, add a base layer that has a larger initial extent (ideally the same base layer that you might have used in ArcGIS Pro to create the .MMPK). Then after you set the MapView.Map property, remove the base layer. The layers in the .MMPK will display at the extent as they were originally created in ArcGIS Pro. The unintended side effect of this workaround is that you can now zoom out past the original extent that was specific in ArcGIS Pro (but at least you can see all of the data).
Here is some sample code to demonstrate the workaround:
string thePathToMMPK = "C:\\Temp\\CurrentDisplayExtent.mmpk";
MobileMapPackage theMMPK = Await MobileMapPackage.OpenAsync(thePathToMMPK);
IReadOnlyList<Map> theMaps = theMMPK.Maps;
Map myMap = theMaps[0];
var serviceUri = new Uri(http://services.arcgisonline.com/arcgis/rest/services/Canvas/World_Light_Gray_Base/MapServer);
ArcGISMapImageLayer baseLayer = new ArcGISMapImageLayer(serviceUri);
myMap.Basemap.BaseLayers.Add(baseLayer);
MyMapView.Map = myMap;
LayerCollection myLayerCollection = MyMapView.Map.Basemap.BaseLayers;
myLayerCollection.RemoveAt(0);
I hope this helps.
-Don
