Thanks for contacting support.
We have tested the scenario using following code snippet and were able to replicate the issue which you have mentioned. We have logged this issue as PDFNET-44655 in our issue tracking system. We will further investigate the issue in details and keep you posted with the status of its correction. Please be patient and spare us little time.
Document doc = new Document(dataDir + "Public_EPoststelle_Hauptdokument_2.pdf");
doc.Convert(new MemoryStream(), PdfFormat.PDF_A_1B, ConvertErrorAction.Delete);
doc.Save(dataDir + "output_pdfa.pdf");
We are sorry for the inconvenience.