The problem is related to all controls that use the EI. It can also be reproduced using the EI directly as shown with the following code:

ei.ExportStructure structure = new ei.ExportStructure();
ei.Table table = new ei.Table();
structure.Tables.Add(table);
ei.Cell firstCell = table.Cells[1,1];
firstCell.Colspan = 2;
firstCell.Style.Font.Bold = true;
firstCell.Style.Font.Size = 15;
firstCell.Value = "TEST";

ei.XlsBiffRenderer renderer = new ei.XlsBiffRenderer(structure);
byte[] output = renderer.Render();
string fileName = Server.MapPath("~/test.xls");

using (FileStream fs = new FileStream(fileName, FileMode.Create))
{
    fs.Write(output, 0, output.Length);
}