The JNB call centre do not have full access and absolutely have all sorts of issues seeing seats that are available for sale. This call centre is an absolute disgrace and QF is a disgrace for continuing to use a call centre that still has not been trained properly and given proper system access...
Be a WP or above to not be treated with complete contempt.
Otherwise, file a complaint.
https://www.qantas.com/au/en/support/contact-us/customer-care-feedback-form.html#
There's a chance you'll get a call from a competent agent to sort things out.