Thanks for your replies - all the sectors I’ve selected are all on the A350 planes. The strange things is - when I deliberately chose the SIN-NRT-SIN route on a 777 (no PEY), the price of the premium economy fare went up by $500 and the BNE-SIN/SIN-BNE suddenly gave me the PEY seating option. Still I question why when I’ve chosen premium economy fares, the ‘unavailable’ next-class-down economy seats isn’t an upgradable seat.