We've manually re-applied the required changes via the specific conf setting in the etc/config.php file, then re-deployed, then re-tested ALL countries with out setup in our Dev Store. Yes, that's fixed everything and we've updated the bug ticket too. Thanks
The separate review into the XC5 upgrade procedure, to understand why the etc/config.php file was not updated during previous upgrades, is a separate matter that we'll leave to XC.