Good question
@Matt_01. SQ changed the waitlist system in 2019 for all members and this has thrown its fair share of curve balls since. In theory, SQ is only making Saver waitlists available on flights they think there is a reasonable chance will clear. In the past you could waitlist all the way up until departure, but all waitlists that do not clear at 14 days prior to departure will now be automatically cancelled. In my experience, they do not reappear within this 14 day window. it's also worth noting that inventory is now much more tightly controlled than in the past, so once the allocation of Saver and Advantage seats for a flight is allocated, they are unwilling to expand availability, even on flights that are far from full, which may explain why both Saver and Advantage are waitlisted simultaneously. Strange...but SQ often moves in mysterious ways.
Generally speaking, waitlists should clear according to status, but I don't have any anecdotal evidence to prove that this does/does not work. That said, my advice would be to call and ask the reservation agent to send a chaser to the load controller. I've had reasonable success using this tactic over the years, and I understand it is available to all passengers regardless of status. The catch is that unlike QF, this is done offline once you finish the call and if the request is unsuccessful, you will not receive any communication that your request hasn't been successful.