I took a number of QR flights in summer 2023, with the flight in question operated by an ex-CX 77W

certain tickets allow seat selection on standard seats (i.e. excluding exit rows, bulkhead rows, any seats on A380 upper deck, the “preferred zone” which is the front few rows of economy class and ex-PEY seats on certain 2nd hand aircrafts), and even at online check-in the free seat selection includes standard seats only (as opposed to some other carriers which allows free seat selection on whatever seats remaining, including extra-legroom ones)

in the unlikely scenario that all standard seats have already been taken, QR has no choice but to provide a preferred seat for free (as you need to be seated onboard anyway!
if you’d like to change to another seat, there will be an extra cost as you’re treated as voluntarily choosing a preferred seat

i’m mentioning ex-CX 77W as this aircraft configuration contains a considerably large proportion of economy class seats designated as “preferred”