Yes I've noticed this too, as have others, and it's a relatively new development that coincided with changes to the Asia Miles on 22 June 2018 (see their forum). There are probably 2 sides to this - a legitimate one being married segments, in that many airlines (including BA) issue some redemptions as married segments, so two flights=good; one flight=bad. Behind this is that the main revenue buckets can have different availability on married segment routes for marketing purposes, and BKK-HKG is known to be in this category. There is nothing you can do about that, the shown inventory in this scenario is what it is. The other aspect is that sometimes the frequent flyer scheme - BAEC in this case - doesn't pick up fare inventory after internal changes at the issue inventory end (CX in this case), whereas connections continue to work since typically they aren't messed about so much in reorganisations. I see a similar issue with Air Italy at the moment, BAEC shows point to point, IB+ only shows connections, and Air Italy was of course reconfigured from Meridiana.

The second area tends to get sorted eventually, I've no doubt BAEC corrected Air Italy faster than IB+ for example. In the case of CX at one stage there was virtually no availability point to point, now routes like HKG-TPE are back to normal. And though we often see questions in this area we unfortunately rarely get data points or experiences of people who do ring up and find availability not available online.