I only have one data point, being new to SQ/KF, but my experience with waitlisted awards automatically clearing is pretty bad:
I wanted to travel JFK-SIN on Wed 11/24 (VTL flight) but there was no Saver availability, only could waitlist for Advantage. So I waitlisted for Saver on Tues 11/23 (suboptimal but could make it work). I checked a week later and Saver awards on both days were wide open for immediate booking! But my 11/23 was still "waitlisted"
Of course, I immediately canceled the waitlisted segment and booked my preferred timing for a Thanksgiving weekend trip to SIN.
I'm guessing SQ initially zeroed out (or severely capped) award inventory on a bunch of VTL flights in this case. But near term demand, at least on this route, didn't materialize. No idea why they don't auto clear though. Definitely worth checking regularly or setting EF alerts.