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.