I believe this is specifically an issue with FJ right now; it seems like some sort of IT glitch between AS and Fiji and is distinct from other phantom availability issues I've seen in the past. I've been chasing U-class space from NAN-SYD on FJ for the last 2 weeks where it "disappears" when they try to add the segment to the flight but continues to show as available on the website (AA/AS/BA/etc.) and shows up as available again to the agents if I call back in a couple of hours later.

My (limited) understanding is that AS adds a new flight segment to the PNR in SABRE, which more or less causes a message to be sent to FJ asking for the new segment to be confirmed. In this case, the agents told me that the new segment came back as unconfirmed, and there really isn't much they can do in that case, but in their experience typically it means the space is no longer available. This happened even with them manually attempting the changes directly in SABRE instead of via their UI.

That said, it seems persistence paid off. When I called this morning, they successfully changed that segment from Coach to Business. It wasn't clear what (if anything) she did differently this time, but it was a frustrating experience since no one can tell you why it's happening. Realistically I'm sure the phone reps don't know and don't have direct contact with anyone that would be able to find out.