Thank you for replying, Siya. 

1. you mean like xlsxwriter instead of openpyxl? The python code was delivered by another team. It's hard for me to try with different approaches and debug. : /

2. So I could also try the BOM removal on the entire Excel file or each XML file by another extension?

Please see if you can remove the BOM on the entire excel.

I tried with the following script in different versions, somehow the BOM elements (UTF-16 LE BOM (FF FE) at position 26478 within sheet.xml and UTF-16 BE BOM (FE FF) at position 35684 within the same sheet.xml file. Both are embedded in the XML content, not in Binary Data) could not be removed. Do you have a clue?

public void MssremoveBomFromExcel(byte[] ssinputExcel, out byte[] ssoutputExcel)

    {

        try

        {

            using (MemoryStream outputMemoryStream = new MemoryStream())

            {

                using (MemoryStream inputMemoryStream = new MemoryStream(ssinputExcel))

                {

                    using (ZipArchive inputArchive = new ZipArchive(inputMemoryStream, ZipArchiveMode.Read, true))

                    {

                        using (ZipArchive outputArchive = new ZipArchive(outputMemoryStream, ZipArchiveMode.Create, true))

                        {

                            foreach (var entry in inputArchive.Entries)

                            {

                                var outputEntry = outputArchive.CreateEntry(entry.FullName);

                                using (var inputEntryStream = entry.Open())

                                using (var outputEntryStream = outputEntry.Open())

                                {

                                    byte[] entryBytes = ReadAllBytes(inputEntryStream);

                                    byte[] cleanedBytes;

                                    if (IsTextFile(entry.FullName))

                                    {

                                        string encoding;

                                        cleanedBytes = RemoveBomAndReencode(entryBytes, out encoding);

                                        Console.WriteLine($"Processed {entry.FullName} with encoding {encoding}");

                                    }

                                    else

                                    {

                                        cleanedBytes = entryBytes;

                                    }

                                    outputEntryStream.Write(cleanedBytes, 0, cleanedBytes.Length);

                                }

                            }

                        }

                    }

                }

                ssoutputExcel = outputMemoryStream.ToArray();

            }

        }

        catch (Exception ex)

        {

            throw new Exception("Failed to remove BOM from Excel content.", ex);

        }

    }

    // Method to remove BOM from a byte array and re-encode if necessary

    private static byte[] RemoveBomAndReencode(byte[] bytes, out string encoding)

    {

        encoding = "UTF-8"; // Default encoding

        // Detect and handle UTF-16 LE BOM

        if (bytes.Length >= 2 && bytes[0] == 0xFF && bytes[1] == 0xFE)

        {

            encoding = "UTF-16LE";

            string content = Encoding.Unicode.GetString(bytes.Skip(2).ToArray());

            return Encoding.UTF8.GetBytes(content);

        }

        // Detect and handle UTF-16 BE BOM

        else if (bytes.Length >= 2 && bytes[0] == 0xFE && bytes[1] == 0xFF)

        {

            encoding = "UTF-16BE";

            string content = Encoding.BigEndianUnicode.GetString(bytes.Skip(2).ToArray());

            return Encoding.UTF8.GetBytes(content);

        }

        // Detect and handle UTF-8 BOM

        else if (bytes.Length >= 3 && bytes[0] == 0xEF && bytes[1] == 0xBB && bytes[2] == 0xBF)

        {

            encoding = "UTF-8";

            return bytes.Skip(3).ToArray(); // Already in UTF-8

        }

        // If no BOM detected, return the original byte array

        return bytes;

    }

    // Method to determine if a file is text-based

    private static bool IsTextFile(string entryName)

    {

        return entryName.EndsWith(".xml", StringComparison.OrdinalIgnoreCase) ||

               entryName.EndsWith(".rels", StringComparison.OrdinalIgnoreCase) ||

               entryName.EndsWith(".txt", StringComparison.OrdinalIgnoreCase);

    }

    // Method to read all bytes from a stream

    private static byte[] ReadAllBytes(Stream stream)

    {

        using (MemoryStream ms = new MemoryStream())

        {

            stream.CopyTo(ms);

            return ms.ToArray();

        }

    }

    // MssremoveBomFromExcel

Have followed another approach - copy all the sheets to new excel using the Nuget package "NPOI" .  Not  very efficient approach. Attached the code for your reference. Its works for me. 

Indeed! It also worked by my side, the excel file is no longer opened with Protected View and can be directly bootstraped into OutSystems. Many thanks, Siya!