I was in a similar boat (OWS, CX paid booking, most flights on QR) where my seat assignments weren't sticking after 2-3h. If I would select "normal" seats they would be kept, but any preferred/exit/comfort seats where not, which I guess was the hint that my OWS status wasn't read correctly. I messaged QR on Twitter, and they removed my FF number, a few minutes later I re-entered it and voila! the changes are now staying, so many thanks for the hint.