Have you verified it in ExpertFlyer? Maybe there's indeed no D available. JL has really tightened up D availability on a lot of routes this summer, I have one to BKK in 2 weeks that I really wanted to change to somewhere further south, but nothing available on a JL code except CGK which they (QF) won't let me do since I had a stopover in CGK already. They admit that there's no rule against it, just they can't do it. Yesterday there was D1 to KUL but of course they didn't check it until today and JL removed the D already and now only J and C are available.

Having said that, I think JL has done a pretty solid job of selling premium cabins this summer to SE Asia because quite a few routes are even low on J inventory for the week I'm checking. .