In a multiple series scenario if one series gets removed, an internal axis flag is incorrectly reset resulting in the horizontal axis displaying only one label when SmartLabelsMode is used. One way to work-around this is to add an empty dummy series after removing a real series:

            this.chart1.Series.Remove(EmptyDummySeries);
            if (this.chart1.Series.Count > 0)
            {
                this.chart1.Series.RemoveAt(this.chart1.Series.Count - 1);
                if (this.chart1.Series.Count > 0)
                {
                    this.chart1.Series.Add(EmptyDummySeries);
                }
            }