I suspect the issue here is the 'base fare must be equal or higher value than the original base fare' rule is in play. QF reinstated this rule last year I believe for tickets issued on/after 1st Oct.
If they originally paid for a higher fare and want to use the credit for a cheaper date later, then they get stuck with paying the higher fare.
Assuming that this is the scenario, then QF hasn't done anything wrong.
It's listed here:
https://www.qantas.com/au/en/manage-booking/credit-vouchers.html
VA has never cared about the new flight being equal or higher value, both now and pre-covid. Their travel bank system, a feature of Sabre, is very good.
SQ you can use the credit across two bookings.
BA and AA you can use the credit across multiple bookings.
QR, JL and NH will refund all tickets, regardless of reason, in full.
LH/LX/OS/SN require equal or higher value unless it's a flex fare then you're allowed to go down in value.