I noticed this too. I had booked my Outbound to Oz no issues. Now attempting to book my return and Qatar is showing availability but at this stupid *2 Avios rate miles!

I have noticed that their algorithm seems to change subsequently so guess I've got to be patient until 335 days before departure. Big change from 180000 to 360000 for the one way journey back from Perth in Business!

It is frustrating but I have found that the QR taxes are cheaper than what BA usually want.

However domestic flights in Australia on Qantas I think are remarkable bang for buck when booking using Avios on BA, given the cost of air fares down under.

If I cannot back from Perth to BHX then I can alternately fly back from DPS or SIN or alternately fly into another UK / European city.

That's my game plan currently