MapPolyline (Informationlayer) or PolylineData (VisualizationLayer) disappear on higher zoom levels. The origin of this issue is that Silverlight has limitation for the size of the polyline objects. Once this limit is reached, the calculations are wrong and the polyline is displayed incorrectly. Possible workaround could be adding multiple smaller polylines / lines instead of one big polyline with many intermediate points. The XAML team has recently reviewed this issue and will not be addressing it as at this time the team is focusing on the bugs impacting the highest number of developers. If you have encountered this issue and it is blocking for your work please contact us through the support ticketing system with details on your setup and a reference to this item.