Pretty simple answer - the .env file was not showing the correct primary site i.e.
should have been
very embarrassingly for me, this error was clearly displayed in the console log of the live preview site i.e.
navigate to the live preview page
enter dev tools
view console for the ...
Whenever I have an issue with live previews between sites, I have found it to be an issue with my security headers and/or .htaccess file. Live preview uses an iframe, so if your site blocks iframes via security headers, then you obviously would not see its contents.
Check the console output via dev tools in your browser, and also check the logs under ...