Currently, when you open an APRX and go to Export Layout, the Accessibility PDF Metadata is for the last PDF you exported. The APRX did not retain the Accessibility PDF Metadata from the last time you exported the layout in that APRX. The need to enter the Accessibility PDF Metadata again is time consuming and can lead to errors and discrepancies in the Accessibility PDF Metadata between exports of the same layout.
Attached is documentation provided to Esri detailing the behavior. Esri was able to reproduce the detailed behavior and put in an Enhancement (ENH-000182509) for it but it needs more support so please give this ArcGIS Idea a Kudos to upvote it. Thank you!
