Asiana Club - HKG in 747/74E [First Class] - why so full?

As I was trying to book a reward segment in ICN-HKG in F in April, I realised that there isn't actually any date in April that it is available for a reward. Then I moved to check in Expertflyer, and it appears that roughly half of the days the flight is showing as full in F, in both directions.

Now, I realise that people do in fact pay for F, but I'm shocked that flights on so many days in April are zeroed out in F. Is there a particular reason for this? Do you think they've blocked F and stopped selling it, or is it genuinely all sold out?

I might give OZ a call tomorrow to see what's going on, but anyone else have any ideas about the strange popularity of ICN-HKG and v.v. in F?

PS. Also, looking on the ANA tool, some days it shows an "x" which means the cabin exists but zero availability, whereas other days it shows "-" which means the computer doesn't think the F cabin exists on that flight. Yet another oddity, it seems!

also keep in mind that they don't sell F on some of the 747's running this route, but reserve the seats for those with C tickets.....

I've since found out that a lot of the dates have now been switched to A330 and B777, which obviously has no F. Maybe they blocked it in order to switch the aircraft...

But a random scattering of dates in April still seem to have the F cabin offered - I wonder whether these will change to a 2-class equipment as well, or stay 3-class?

PS. Do OZ open up more rewards seats in F closer to departure date?