To reproduce:
Merge some cells the insert a row:
private void button4_Click(object sender, EventArgs e)
{
    string fileName = @"..\..\exportresult1234.xlsx";
    WorkbookFormatProvidersManager.RegisterFormatProvider(new XlsxFormatProvider());
    IWorkbookFormatProvider formatProvider = WorkbookFormatProvidersManager.GetProviderByName("XlsxFormatProvider");
    FileStream fileStream = new FileStream(fileName, FileMode.Create, FileAccess.ReadWrite, FileShare.ReadWrite, 32 * 1024, false);
    Workbook workbook = new Workbook();
    Worksheet worksheet = workbook.Worksheets.Add();
    using (fileStream)
    {
        for (int rowIndex = 0; rowIndex < 20; rowIndex++)
        {
            for (int colIndex = 0; colIndex < 20; colIndex++)
            {
                if (rowIndex % 4 == 0)
                {
                    worksheet.Cells[rowIndex, colIndex, rowIndex, colIndex + 20].Merge();
                    worksheet.Cells[rowIndex, colIndex].SetValue("MERGED CELL");
                    break;
                }
 
                worksheet.Cells[rowIndex, colIndex].SetValue("text " + rowIndex + colIndex);
            }
        }
 
        if (worksheet.Rows.CanInsert(0, 1))
        {
            RowSelection selection = worksheet.Rows[0];
            selection.Insert();
        }
 
        CellIndex from = new CellIndex(0, 0);
        CellIndex to = new CellIndex(0, 16);
        CellSelection selection1 = worksheet.Cells[from, to];
        selection1.Merge();
        selection1.SetValue("Header TEXT MMMMMMMMMMMMMMMMMM");
        selection1.SetHorizontalAlignment(Telerik.Windows.Documents.Spreadsheet.Model.RadHorizontalAlignment.Center);
        selection1.SetFontSize(24);
 
        formatProvider.Export(workbook, fileStream);
    }
}

Workaround:
Reopen the document and then insert the row.
This item is migrated to the Telerik Document Processing portal: 
http://feedback.telerik.com/Project/184/Feedback/Details/190216 
Please use the new item for commenting, voting and subscribing instead of this one.