Hi again epicdevspace,
I’ve had a response that provides the reason for the problem. When the client calls TEST_SHORTCODE.php directly, MemberPress knows its a client (perhaps via cookie or session variable) and can respond. However, if the client calls the CALL_SHORTCODE.php script and its the script that calls TEST_SHORTCODE.php then the call is coming from the server not the client.
So I appear to have an explanation but no solution to my problem which will need a rethink.
Thanks for taking the time to consider it. I’ll mark the question closed.
Regards,
-Jason