chrisgdclarke I can confirm that the edit function does work on the Protostar template, but not on the T3 Blank as described.
As apparently seen, Default Joomla template has been uninstalled. For troubleshooting purposes, Would suggest to re-install Protostar by going to Extensions > Manage > Discover > Protostar (search) > Install and then, check if the same issue persists after assigning default template for specific menu item
Also, Wished to confirm if the relative issue did happen with previous version of Joomla with T3 blank?