Its maybe the class of travel in premY that is not eligible.
Ie, the class exist under BA, but QF does not have it in its "partner" earn T&C.
I haven't had an indepth look.
Even though BA might have its premY class, and if QF does not have an equivalent, its zilch, or you could put an appeal.
Do you know the sub class of your travel?
Its the A to Z code, not alpabet A/Z themselves, but the ones in between.
+++
Edited to add:
Singapore to Sydney
Airline
British Airways (BA)
QANTAS POINTS
STATUS CREDITS
- Discount Economy
you will earn1,000Qantas Points
and15Status Credits
- Flexible Economy
you will earn4,000Qantas Points
and30Status Credits
- Business
you will earn5,000Qantas Points
and60Status Credits
- First
you will earn6,000Qantas Points
and90Status Credits
SHOW LESS
The requested frequent flyer earn category is not offered, the available category list has been
So, you can see, if had flown QF premY, you would earn, but as you have flown BA, under QF SC and points earn, its not mentioned.
You should put in an appeal, should mention you "ought" to get at least Y earn partner level points earn, and partner level QF SC earn, at the Y earn.
The italics is from the QF points earn calculator sub website.
***
QF can also be quite "mean" on routes where they fly to and from, and for which a partner flies to and from.
Not only BA fliers are treated meanly by QF, MH fliers are the same.