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I could see why supply of Bandai's 30 Minute Missions is still inadequate to meet demand as of late, because production schedules are pretty much set in stone 4 months in advance.
That means when Bandai confirmed the success of the June and July waves at around August, they could only decide to ramp up the production numbers of the December wave.
It looks like the December wave's release was condition upon the success of the first two waves. That is why preorders are still open after a week.