Flattening a PDF with Siganture Field

Hi,
I have dynamic XML form, which has 2 signature fields. In my process I flatten the document using transformPDF. But if one of the signature fields are signed then, the PDF cannot be viewed in Acrobat.
I logged the PDF properties of both below.
Signed PDF's properties after flattening:
isPDFDocument=true isPDFPackage=null PDFVersion=1.7 hasAttachments=null hasComments=false requiredAcrobatVersion=8.0 formType=NotAForm isAcroForm=false isFillableForm=false isXFADocument=false XFAVersion=null
Empty PDF's properties after flattening:
isPDFDocument=true isPDFPackage=null PDFVersion=1.7 hasAttachments=null hasComments=false requiredAcrobatVersion=8.0 formType=NotAForm isAcroForm=false isFillableForm=false isXFADocument=false XFAVersion=null
What is the reason for this difference?
Thanks