Problem can be seen in the following code (Line series for comparison):

    Line series1;
    FastLine series2;

    private void InitializeChart()
    {
      tChart1.Aspect.View3D = false;
      series1 = new Line(tChart1.Chart);
      series2 = new FastLine(tChart1.Chart);


      series1.LinePen.Width = 3;
      series1.Stairs = true;
      series1.InvertedStairs = true;
      series2.Stairs = true;
      series2.InvertedStairs = true;

      series2.TreatNulls = TreatNullsStyle.DoNotPaint;

      Random rnd = new Random();

      for (int i = 0; i < 20; i++)
      {
        double y = rnd.NextDouble();
        if (i > 8 && i < 12)
        {
          series1.Add(i, 0, Color.Transparent);
          series2.Add(i, 0, Color.Transparent);
        }
        else 
        { 
          series1.Add(i, y);
          series2.Add(i, y);
        }
      }
    }

    private void button1_Click(object sender, EventArgs e)
    {
      series1.InvertedStairs = !series1.InvertedStairs;
      series2.InvertedStairs = !series2.InvertedStairs;
    }

    private void button2_Click(object sender, EventArgs e)
    {
      series1.Stairs = !series1.Stairs;
      series2.Stairs = !series2.Stairs;
    }