I modified one of the standard WPF samples (RenderSimpleMarkers) to create a line graphic instead of a point graphic e.g:
replaced:
Graphic graphicWithSymbol = new Graphic(centralLocation, simpleSymbol);
with:
Graphic graphicWithSymbol = CreateSimpleLine(), where CreateSimpleLine is the following procedure:
public static Graphic CreateSimpleLine()
 {
 MapPoint point1 = new MapPoint(-79.497238, 8.849289, SpatialReferences.Wgs84);
var newPoint1 = (MapPoint)GeometryEngine.Project(point1, SpatialReferences.WebMercator);
var cartoLineSym = new SimpleLineSymbol(SimpleLineSymbolStyle.Solid, Colors.Black, 1);
MapPoint point2 = new MapPoint(-80.035568, 9.432302, SpatialReferences.Wgs84);
 var newPoint2 = (MapPoint)GeometryEngine.Project(point2, SpatialReferences.WebMercator);
 var aBuilder = new PolylineBuilder(new MapPoint[] { newPoint1, newPoint2 });
var aGraphic = new Graphic(aBuilder.ToGeometry(), cartoLineSym);
 return aGraphic;
 }
If I change the SimpleLineSymbolStyle to other than solid everything works OK but with the solid pattern the program throws!
We are using this to draw a solid line with throwing any errors.
The polyline passed in is created using a List<MapPoint>, such as var polyline = new Polyline(mapPtList);
Maybe there is an issue with the Geometry created by the polyline builder.
/// <summary>
 /// Creates the line symbol.
 /// </summary>
 /// <param name="polyline">Polyline.</param>
 public static Graphic CreatePolylineSymbol (Polyline polyline)
 {
  //Create the symbol and add the graphic to the map
  var lineSymbol = new SimpleLineSymbol () {
  Width = 5,
  Style = SimpleLineSymbolStyle.Solid
 #if !__IOS__
 #if WINDOWS_UWP
           ,Color = Windows.UI.Colors.Cyan
 #else
           ,Color = System.Drawing.Color.Cyan
 #endif
 #endif
  } ;
 
  //Create the graphic and add to the map
  Graphic lineGraphic = new Graphic (polyline, lineSymbol);
  return lineGraphic;
 }
Can't be an issue with the geometry because it works as expected for other line styles...
Good point!
I tried your code as-is for creating geometry and symbol, added this line to see the graphic. SimpleLineSolid.Style worked. Also tried in on Forms.Android.
MyMapView.SetViewpoint(new Viewpoint(aGraphic.Geometry.Extent));Since I couldn't repro, I tried the following to iterate through SimpleLineSymbol styles and use SketchEditor to draw freehand (gesture: touch down, drag and up).
foreach (SimpleLineSymbolStyle lineStyle in Enum.GetValues(typeof(SimpleLineSymbolStyle)))
{
    var symbol = new SimpleLineSymbol(lineStyle, Colors.Cyan, 5d);
    try
    {
        var geometry = await MyMapView.SketchEditor.StartAsync(SketchCreationMode.FreehandLine, false);
        overlay.Graphics.Add(new Graphic(geometry, symbol));
    }
    catch (TaskCanceledException)
    {
    }
    catch (Exception ex)
    {
        System.Diagnostics.Debug.WriteLine(ex.Message);
    }
}Every style worked and Null style is the only thing that did not render as expected.
GraphicsRenderingMode.Dynamic(default) or Static did not seem to make a difference in this case. Am I missing something in your repro code?
You can take the following code and paste it in the RenderSimpleMarkers.xaml.cs, sample provided in from ArcgisRuntime.WPF.Samples. This draws a red line across the Panama Canal, if it's style isn't SOLID of cource!
public partial class RenderSimpleMarkers
 {
 public RenderSimpleMarkers()
 {
 InitializeComponent();
// Create the UI, setup the control references and execute initialization 
 Initialize();
 }
private void Initialize()
 {
 // Create new Map with basemap
 Map myMap = new Map(Basemap.CreateImagery());
// Create initial map location and reuse the location for graphic
 MapPoint point1 = new MapPoint(-79.497238, 8.849289, SpatialReferences.Wgs84);
var centralLocation = (MapPoint)GeometryEngine.Project(point1, SpatialReferences.WebMercator);
Viewpoint initialViewpoint = new Viewpoint(centralLocation, 1000000);
// Set initial viewpoint
 myMap.InitialViewpoint = initialViewpoint;
// Provide used Map to the MapView
 MyMapView.Map = myMap;
// Create overlay to where graphics are shown
 GraphicsOverlay overlay = new GraphicsOverlay();
// Add created overlay to the MapView
 MyMapView.GraphicsOverlays.Add(overlay);
// Create a simple marker symbol
 SimpleMarkerSymbol simpleSymbol = new SimpleMarkerSymbol()
 {
 Color = Colors.Red,
 Size = 10,
 Style = SimpleMarkerSymbolStyle.Circle
 };
// Add a new graphic with a central point that was created earlier
 Graphic graphicWithSymbol = CreateSimpleLine(); //new Graphic(centralLocation, simpleSymbol);
 overlay.Graphics.Add(graphicWithSymbol);
 }
 
 public static Graphic CreateSimpleLine()
 {
 MapPoint point1 = new MapPoint(-79.497238, 8.849289, SpatialReferences.Wgs84);
 var newPoint1 = (MapPoint)GeometryEngine.Project(point1, SpatialReferences.WebMercator);
 var cartoLineSym = new SimpleLineSymbol(SimpleLineSymbolStyle.DashDot, Colors.Red, 5);
 MapPoint point2 = new MapPoint(-80.035568, 9.432302, SpatialReferences.Wgs84);
 var newPoint2 = (MapPoint)GeometryEngine.Project(point2, SpatialReferences.WebMercator);
 var aBuilder = new PolylineBuilder(new MapPoint[] { newPoint1, newPoint2 });
var aGraphic = new Graphic(aBuilder.ToGeometry(), cartoLineSym);
 return aGraphic;
 
 }
 }