Okay, the way the chapters are arranged was really confusing, so since I figured it out I'll describe it here. Basically, the first chapter was published in two different volumes, both oneshot collections, with only the first story in common. Each group scanned a separate set of stories, but they're labelled in a way that looks like alternate chapters. For example, rather than two versions of volume one chapter two, we actually have two separate stories, that were both in the place of chapter two in different volumes.
Actually, after typing it out like that I see how dumb it is and I'm going to report the manga and ask a mod to mark the stories from the later release as volume 2.
Edit: just realised that the titles of the chapters help clear confusion for anyone who isn't on mobile and can actually see them. That's more of a mangadex issue, I hate not having chapter titles.