OK, my apologies, I misunderstood your point before. That date is going to be a tricky one for redemptions due to the alignment of the end of the school holidays, the end of the general holiday season for Europe and the UK bank holiday. However I can see the point you are making: if one looks at the AMS-HKG service on Cathay, availability is pretty good in Business, and quite good in Premium Economy. But relatively little in Economy itself, and when I found some dates in mid November it then refused to price it up, whereas Premium Economy is fine.

So it may be a BA glitch, or the notorious Cathay phantom availability. I would be tempted to go for the Premium Economy option, paying more cash to reduce Avios if you were low on the latter, and then switch to Economy if and when it shows up.