The reciprocal rules for AC access for QP members, and QP access for AC members, have seemingly been in flux recently.
According to Qantas QP members have to be on a QF/JQ flight number to access AC:
Eligibility and Access to The Qantas Club Lounges | Qantas
"Each time you travel on a Qantas or Jetstar flight number you will have access to:
- All Qantas Club lounges when travelling within Australia
- All Qantas International Business Lounges when travelling overseas
- Emirates Business Lounge in Dubai
- American Airlines® Admirals Club lounges in London and other locations - visit www.aa.com for lounge locations"
This is also confirmed for particular lounges, eg LAX T4 AC ... QP access only if QF/JQ flight number.
Lounge Locations | North America | Los Angeles International Airport Terminal 4 | Iternational Business
AC access as you say used to be universal in US AA clubs for QP, for as long as I can remember (20 years plus ?). The restricted access conditions to AC that have operated for several years at LHR for QP members now seem to have been extended to all ACs.
So ... supposedly enhanced partnership between QF and AA recently ... but restricted QP privileges ... hmmm.