Missing field value after flatten

PDDocument pdfDocument = PDDocument.load(new File("a.pdf"));
pdfDocument.setAllSecurityToBeRemoved(true);
PDDocumentCatalog docCatalog = pdfDocument.getDocumentCatalog();
PDAcroForm acroForm = docCatalog.getAcroForm();
if (acroForm != null) {
  PDField field = (PDField) acroForm.getField("nas");
  field.setValue("test01");
  field = (PDField) acroForm.getField("nom");
  field.setValue("test02");
  acroForm.flatten();
}
pdfDocument.save("output.pdf");
pdfDocument.close();