I have to say CI is much more generous in this regard, especially on TPE-SHA where they also use 77W/747 but make those PE/upper deck J seats available to Economy passengers, instead of charging them Premium Economy.
Indeed Taiwan-China flights are just so popular and constantly overbooked. For those who really need to travel on specific dates, you either buy Premium Economy or leave it. Otherwise Premium Economy tickets on a <2 hour flight is totally absurd...