I've noticed that there are quite a few oneshots on this website that are listed under a Volume 0, like for example: https://mangadex.org/title/b833f337-e87f-4c4f-88cf-c405778c277b/08-05-no-hengao-san.
Now usually I wouldn't make a big deal out of this, but this leads apps like Tachiyomi or...