Digital Signature does not validate
I have a PDF that I created on pdffiller.com that has a signature field. After signing the signature field through Telerik's PdfProcessing library, Acrobat does not show the PDF as being signed. Although, Foxit PDF Reader and Telerik's WPF PdfViewer do show the document as being signed.
I have attached the before and after PDFs.
Your documents have the NeedAppearances flag set to true. This makes any signatures therein questionable as it instructs any PDF processor to "construct appearance streams and appearance dictionaries for all widget annotations in the document". Thus, chances are that the signer saw something different when signing than another person viewing the signed document, in particular if a different viewer program is used.
If you set that flag to false (or don't add it at all), Adobe Acrobat recognized the signature.