There is a problem with your argument: You can't expect any manga to be selling like 100K first volume to compensate anything. The most recent manga that made it that far on volume 1 was Ruri Dragon, since it sold 120K on its first volume. And that was 4 years ago. Funny enough is that Ruri...
I understand the reason why Kagurabachi could be on the bottom. But to be on the dead last for two issues and second to last for one is really strange. Worse enough is to have no anniversary cover.
Roku Volume 1 is at 299.
We will have to wait, but I don't think Under Doctor is gonna do well in sales as it did in volume 2. But I don't think it is at risk for now.