Unfortunately there's a weird "dotting" effect on the actual manga pages. This was exactly how the author uploaded the images though, and I intend to keep it that way. It's maybe an intended effect that looks fine when printed, but sticks out on the very high resolution raws? The effect seems harder to notice when the image is made smaller.
I would be very interested in someone telling me if there's a way to process these to look better though, even though I intend to leave it as is for now.
Note: I've noticed this problem before on his official digital releases but this is by far the worst looking one.