Watch it guys!
I have just had exactly the same problem, in this case, the HKG-SYD leg of a multileg is in the Economy Cabin. Cathay being the carrier for that part.
As with others, front page is specified "Business" and throughout the booking, it's showing as "Business Flex" (not even saver!).
I have booked this ticket many times before, previously it always booked into CX J class without a problem. Finnair strikes again, pfft.