@
barracuda93 as you are based in Taiwan, are you booking TPE-HKG-xxx flights? There is a lot of availability when you book this kind of flight with a married segment.
But if you are looking at HKG-BKK or HKG-SIN for example, it becomes much harder to find flights.
Actually I avoid booking combinations of flights in a single itinerary with BAEC for several reasons: better flexibility (I can cancel or change just one flight without affecting another) and if I fly via Hong Kong, I usually prefer having a day there.
But yeah, in certain instances I have noticed this weird problem with Iberia and perhaps BAEC. As I remember, it was when I looked for flights between Hong Kong and Jakarta. Never had problems booking HKG–TPE, HKG–BKK or HKG–MNL though.
I also typically use Avios to book flights from Taiwan to Japan or Korea.