Maybe it is just me, but adding another segment as waitlist to an existing PNR has never worked online to my recollection. Not sure what SQ agents are trying to advocate?

If they want people to create a new, fresh PNR with only the waitlist segment (s) on it then fine, but that means one needs to have the full mileage in the account to confirm the booking later on IF the waitlist clears.

That is in contrast to having to call KF redemption agent to confirm the waitlisted segment (after it clears) under the same PNR, remove the original segment and collect the difference in mileage (and maybe some tax and fees).

I guess they don't like this extra work any longer and having people make new waitlist bookings means more miles need to be transferred into KF (possibly). YMMV.