I stumbled across a work around purely by accident but to cut a long story short:
Problem is that since the upgrade database url points to an unsecure http file, my server then wants to redirect to the secure url - resetting the session - requiring an admin login (not sure why this is since automatic SSL redirects have been disabled). Using the upgrade url, ...
Was a solution found for this? I'm having the same problem.
I successfully tested the upgrade with a cloned staging site without a hitch in the upgrade process.
By following the CiviCRM Wordpress upgrade guide: https://docs.civicrm.org/sysadmin/en/latest/upgrade/wordpress/
However, as soon as I attempted on the main site ...