There are particular issues with SQ right now. They are devaluing KrisFlyer points on 5 July so a lot of KF members will be booking awards now at the current rates, potentially looking to get the dates changed when for future inventory is released. This has caused a dearth of premium seats at the far end of the calendar, which has led KF members who actually want to fly on those dates having waitlisted for huge blocks of dates in case one of those dates frees up. Plus, I understand SQ may have done a ring around to PPS members to encourage them to block book award capacity at the far end of the calendar. If I were a betting person, I would say that a huge amount of award space has been booked by people not intending to use the flights they have booked/waitlisted and at some future point, those bookings will be released.
Pre-Covid there was usually fairly good availability of premium seats (at least through KrisFlyer) and most did not get snapped up the moment they were released.