I had a similar situation with D seats a few years back when QF100/101 followed the same routing as the new QF25/26. D was available for MEL-LAX via AKL, but D0 for AKL-LAX. I was planning to travel BNE-AKL-LAX on a DONE4, but instead had to go BNE-MEL-akl-LAX.

I was waitlisted for D on AKL-LAX for a few days, but had to drop the waitlist when the ticket was issued since you can only waitlist a sector if you have a confirmed alternative on the same routing.

This is quite common for awards on through flights and happens on flights to LHR and FRA quite regularly.