You use Tileset, right?
This problem is described in the documentation, and most likely will never be solved.
The problem in triangulation of complex surfaces.

You have three ways:
1. Use the «Drawing to texture» mode. In this mode filling works correctly.
2. OnlineMapsTileSetControl.drawingMode = OnlineMapsTilesetDrawingMode.overlay.
But I do not recommend using this way. You get a huge performance degradation.
3. Make your own drawing API. It's pretty easy.