OpenXML not able to get the actual formula text for few cells

We are using openxml nuget package DocumentFormat.OpenXml to read excel cell values.
Our requirement is to read the actual formula text from a cell using "cell.CellFormula" property,but for few of the cells we are getting the actual value instead of formula.
Ex:Say the cell(A10) has the formula : =A5+1  AND A5 has value of "2".
our intention is to read the formula in A10 which is "=A5+1" but instead we are getting the value as "2"
  using (var document = SpreadsheetDocument.Open(filePath, false))
            {
                var workbookPart = document.WorkbookPart;
                var workbook = workbookPart.Workbook;
                var sheets = workbook.Descendants<Sheet>();
                foreach (var sheet in sheets)
                {
                    var worksheetPart = (WorksheetPart)workbookPart.GetPartById(sheet.Id);
                    var cells = worksheetPart.Worksheet.Descendants<Cell>();
                    foreach (var cell in cells)
                    {
                            if (cell.CellFormula != null || cell.CellValue != null || cell.DataType != null)
                            {
                                string cellFormula=cell.CellFormula;

                            }
                    }

                }

            }