ICell.RichStringCellValue.ApplyFont() is applied to all cells with identical values about npoi HOT 2 CLOSED

var redBoldFont = workbook.CreateFont();
redBoldFont.Color = HSSFColor.Red.Index;
redBoldFont.IsBold = true;

for(int i = Convert.ToInt32(numericUpDownSkipRows.Value); i <= workSheet.LastRowNum; i++)
{
    var row = workSheet.GetRow(i);
    if (row == null)
        continue;

    var cellHighlight = row.GetCell(indexHighlight);
    var cellSearch = row.GetCell(indexSearch);

    if (cellHighlight == null || cellSearch == null)
        continue;

    var richTextString = new XSSFRichTextString(cellHighlight.StringCellValue);

    var toSearch = cellSearch.StringCellValue;
    if (String.IsNullOrWhiteSpace(toSearch))
        continue;
    Int32 start = 0;
    while ((start = cellHighlight.StringCellValue.IndexOf(toSearch, start, StringComparison.InvariantCultureIgnoreCase)) != -1)
    {
        var end = start + toSearch.Length;
        richTextString.ApplyFont(start, end, redBoldFont)
        cellHighlight.SetCellValue(richTextString);
        start = end;
    }
}