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.
Dan Hutchins shared this idea
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