I am using version 100.4 of the runtime.
It seems that the SketchEditor's default double-click to finish behavior is different between Android and the other platforms. When sketching a line or polygon on Windows or iOS, the point that is double-clicked to finish the sketch will be part of the final geometry. However, on Android the point that is double-clicked will not be part of the geometry, only the points that we're added already. Here is how I am using the SketchEditor:
await mapView.SketchEditor.StartAsync(SketchCreationMode.Polyline, false);
I'm seeing the same thing in Xamarin.Forms. On iOS and UWP, the double-clicked point is part of the geometry. On Android, double-clicking anywhere ends the sketch.
If there's an option to control this, I'd prefer that iOS and UWP do the same as Android. It really helps when using the magnifier to do a precision sketch.