The digital version of Kenkyuu Koumoku: Seishoku Koui.
Please support the artist: https://www.pixiv.net/en/users/47785 https://alice-books.com/item/show/1995-16 https://alice-books.com/item/list/all?circle_id=1995 https://www.dlsite.com/maniax/circle/profile/=/maker_id/RG52903.html
Having looked at the files more closely since I posted that, I think the author forgot to reduce that one image down to the publication size from the resolution he drew it at. Shrinking it in Photoshop caused the sceentones to resolve the exact same way they look in all the other images.
It's kinda annoying to have found that out since now I want the originals I didn't even know existed. Oh well.