FYI, I'm also seeing GeometryEngine errors on the two test polygons:
Internal error exception
at RuntimeCoreNet.Interop.HandleException(Boolean retVal)
at RuntimeCoreNet.CoreGeometryEngine.Intersects(ICoreGeometry coreGeometry1, ICoreGeometry coreGeometry2, CoreSpatialReference coreSpatialReference)
at Esri.ArcGISRuntime.Geometry.GeometryEngine.Intersects(Geometry geometry1, Geometry geometry2)
Update: If I simplify the polygons, the intersection test works.
Update 2: I just looked at the results of the simplify, and they're bogus.