With recent versions of Acrobat Reader, we found that when opening a document with certification signature (DocMDP signature), There is always the warning indicating "Changes have occurred. Click to validate the signatures" in the signature panel. Click it and then after revalidation the warning is gone.
I have attached the PDF used for the screenshots below.
This also happened on the "Signed Version" opened by right clicking on an already-validated and valid signature and choosing "View Signed Version".
We have seen similiar discussions on the forum but could not find a solution or workaround to get rid of this warning. We have found some PDFs which do not have the warning at file opening but also shows the warning in "View Signed Version". Therefore we suspect that the root cause is in the validation process of Reader.
In addition, we have not found this issue in PDFs that have no certification signature or have two or more signatures (which may include one certification signature). It seems this issue only affect PDFs with certification signatures only.
This is not only annoying, but we already have customers questioning the reliability of PDF signatures. It would also be helpful to know how to workaround this.