To reproduce, use this code:

    Line line;
    private void InitializeChart()
    {
      tChart1.Aspect.View3D = false;
      line = new Line(tChart1.Chart);
      line.FillSampleValues();
    }

    private void button4_Click(object sender, EventArgs e)
    {
      line.XValues.DateTime = !line.XValues.DateTime;
    }

    private void button1_Click(object sender, EventArgs e)
    {
      tChart1.ShowEditor();
    }

and follow these steps:
1) run code "as-is"
2) open chart editor and navigate to "Data" tab
3) close chart editor and click button1
4) repeat 2) and observe that XValues are not DateTime