This is by design in Sign pane. If you add comments to a PDF and "finalize" it from Sign pane by clicking on 'Done signing' button which essentially flattens your PDF document, you wont see any comments in Comment pane. Can you elaborate on what are you trying to accomplish using the sign tool, so that I can suggest a workaround?
We currently receive e-mail bills from a vendor that are in pdf format, we print them out, handwrite notes on them, then pass them around for signature (two-three different people need to comment and sign), pay them, scan the marked up copy back into a pdf and attach it in our accounting program. We were hoping with these new features that each person could make their comments and sign the bill while keeping it in an electronic pdf format.
This can be done using the Sign pane without having to print the PDF/scanning pages after adding comments/signatures.
You can add text comments using the 'Add text' tool from Sign pane, add your signature using the 'Place signature' option where you can choose from 3 different signing options (typewritten signature, hand-drawn signature or importing signature from an image which can be a scanned image of your signature). After one person has added his comments and signature, you can choose to "finalize" the PDF which flattens the PDF so that the comments/signature of this person cannnot be modified by others. The next person can now add his comments/signatures on this PDF again. After all people are done with signing, you can attach the final PDF to your accounting program. This should work seamlessly.
Hope that helps!