Under the WFSLayer.as in the wfst library:
  override public var symbolFunction :Function = function( graphic :Graphic ) :Symbol
  {
   if( graphic != null && graphic.geometry != null )
   {
    if( graphic.geometry is MapPoint )
    {
     return new SimpleMarkerSymbol( "circle", 13, 0xfdd0a2, 0.75, new SimpleLineSymbol( "solid", 0xfd8d3c, 1, 2 ) );
    }
    if( graphic.geometry is Polygon )
    {
     return new SimpleFillSymbol( "solid", 0xfdd0a2, 0.75, new SimpleLineSymbol( "solid", 0xfd8d3c, 1, 2 ) );
    }
    if( graphic.geometry is Polyline )
    {
     return new SimpleLineSymbol( "solid", 0xfd8d3c, 1, 3 );
    }
   }
   return null;
  }
While symbol functions have been deprecated somewhere between 1.1 to 1.3. This I believe is the root of the issue. There may be other unknowns that are creating compatibility issues.
I will attempt to update the wfst library to the 1.3 spec then add it to the flex viewer for a wfs widget to use.