Sign field name is showing as JUNK value

I am facing an issue where the encrypted and signed document is showing the sign field name with junk value.

This is the resultant PDF which is showing sign field name as junk.

If we sign PDF without encrypting the PDF it shows the correct field name. But if we do the encryption first the field name becomes junk. I need help regarding this behavior and how can I resolve this

Following is a code snippet that we are using to Encrypt and then sign the PDF:

public string DigitallySignAndEncryptPDF(string fileToSign, string fieldName, string certificateFile, string certificatePassword, string imageFile, bool isEncrypt)
        {
            string signedPath = "";
            string inputFile = "";

            try
            {
               byte[] pdfBytes = File.ReadAllBytes(fileToSign);
               if (isEncrypt)
                    pdfBytes = ProtectPDF(pdfBytes, "", out bool error);

               X509Certificate2 cert = new X509Certificate2(certificateFile, certificatePassword);

               inputFile = Path.Combine(m_CurrentDirectory, "Input_" + Guid.NewGuid() + ".pdf");
               File.WriteAllBytes(inputFile, pdfBytes);

               signedPath = Path.Combine(m_CurrentDirectory, "Output_" + Guid.NewGuid() + ".pdf");
               int nResult = QP.SignFile(inputFile, m_MasterPassword, "Test Sign Field", signedPath, certificateFile, certificatePassword, m_ReasonToSign, m_LocationOfSign, m_ContactInforToSign);
            }
            catch(Exception ex)
            {
               MessageBox.Show(ex.Message);
               signedPath = "";
            }
            finally
            {
               if (string.IsNullOrEmpty(inputFile) == false && File.Exists(inputFile))
               {
                    File.Delete(inputFile);
               }
            }
            
            return signedPath;
        }

public byte[] ProtectPDF(byte[] pdfToProtect, string password, out bool error)
        {
            error = true;
            byte[] protectedPDF = null;
            try
            {
               int nLoaded = QP.LoadFromString(pdfToProtect, "");
               if (nLoaded == 1)
               {
                    int nProtected = QP.Encrypt(m_MasterPassword, password, 3, QP.EncodePermissions(0, 0, 0, 0, 0,0, 0, 0));
                    if (nProtected == 1)
                    {
                        protectedPDF = QP.SaveToString();
                        error = false;
                    }
               }
            }
            catch (Exception ex)
            {
               ;
            }
            return protectedPDF;
        }

Thanks Ingo for your feedback.

I tried out what you have suggested. But it still shows the junk field name.

Then I tried the same from Debenu Demo app and there it works fine.

The only difference I see is the DLL that is being used in the Demo app and our project.

Demo app is using "DebenuPDFLibraryAX1411.dll" and our project is using "DebenuPDFLibraryDLL1212.dll".

Still trying to figure out if this is the issue or not.