Expertflyer is getting D9 for that flight!
Back in August, I had a similar issue when booking my DONE4 for a MEL-LHR leg, Expertflyer was getting D0 on the day 1 wanted to travel, D4 the day before. (Thu/Fri before a long weekend this March)
CX in Columbo could not get a D seat on either day, even with calling Qantas. As I hate waitlisting I ended up getting a land segment to SYD, then QF29.
(I have since booked a connecting MEL-SYD segment during a recent sale for $88.)