The following code works fine in the XLS file but the XLSX file has missing text.
Workbook wb = new Workbook();
Worksheet ws = wb.Worksheets[0];
TextBox t = ws.TextBoxes[ws.TextBoxes.Add(0, 0, 100, 100)];
t.Text = "Hello World!";
t.Characters(2, 5).Font.Color = Color.Red;
wb.Save("output.xls");
wb.Save("output.xlsx");
Using version 7.3.5.0.