It appears to be a nude filter of an image from one of the artbooks. post #980466 for the original version. I don't yet know the source of the nude filter, so I don't know if it's a third party or actually part of a set, but I'd bank on third party.
E: Earliest occurrence I can find of it (or at least something similar, as this exact image only appears to exist on the boorus) was in late July on 4chan. No references to official work, or anything else for that matter. Probably definitely third party nude filter.
Look closely. Editing artifacts are visible around her neck, her midriff, the underside of her boobs, the corner of her mouth... basically anywhere the swimsuit met the rest of the lineart in the original.
