Right now my conclusion is that it's quasi-phantom availability... The multicity tool or general date picker (top row of flexible non-multicity display) is showing the full extent of the window that Finnair releases awards on Finnair Plus, but that availability is not accessible to QFF until about a week later. The actual Qantas date picker in the initial request or the new Flight Finder appears to be the real extent of QFF access to AY inventory. I'll update this if I manage to get something booked for my dates.
Wow, the AY availability via QFF is a mess. A mix of phantoms and married/divorcing availability. And what Finnair has available to its members bears little resemblance to what QFF can actually do.
For instance, in two calls to QF all classes were 0 for my date for the BKK-MEL leg. The website was showing all classes as bookable with points. On a third call (30mins later), they could see J on that leg and date, but couldn't book it as part of HEL-BKK-MEL, requiring two separate bookings (with resulting unreasonable point cost).
For the following day, the same itinerary was successfully bookable all the way through (one ticket) on the phone, but errored out online when trying there first. Meanwhile, Finnair's own site has no availability on either date in J, but two days ago had availability but only offered as individual segments, while two days earlier it was fine to book all the way.
The saving grace in all this is that each call only involved about 5 mins of waiting! And then unnecessarily long times with the really not very good agents... including being transferred three times.
So I'd say that if you get any errors when trying to (compulsorily) use the multi-city interface to finalise a booking, don't assume it's phantom or will pass... just sigh and call. And insist they check both through-itineraries and individual segments to see what might be going on.