Fill & Sign Bug
It appears that something has changed recently, or Fill & Sign has a bug. If I use Fill & Sign, save, then come back later to finish the document. The fields that I've populated are now read-only. In the past, I could still use Fill & Sign to complete the document and go back to previous filled fields and fix typo's and incorrect data.
Alexander Schlake commented
Are you adding signatures and/or initials to the documents that become read-only? Adding signatures and/or initials will flatten the document and prevent changes to the pre-existing fields and signatures/initials.
Alex from the Adobe Sign team