I analyzed your code (thank you by the way).

It seems I am doing everything right...

Here's a chunk of my code:

public void setChartAppearance(UltraChart ctr)
{
    ctr.SuspendLayout();
    
    // [set some properties in the axes, chart text, labels, etc...]
    
    ctr.LineChart.DrawStyle = this.DrawStyle;
    ctr.LineChart.EndStyle = this.EndStyle;
    ctr.LineChart.HighLightLines = this.HighLightLines;
    ctr.LineChart.StartStyle = this.StartStyle;
    ctr.LineChart.Thickness = this.Thickness;

    ctr.Invalidate();
    ctr.ResumeLayout(false);
    ctr.PerformLayout();
}

I am calling this after I make changes in a property grid.

The strangest part (or not) is that, if I change the draw style or the start/end style, I can see the changes in the chart, but the same does not happen when I change the line thickness or the HighlightLines properties.