@mammaria of all of them, this was one
most likely. Reason being, every single other oneshot was based on a syosetu (JP web novel site) short story and adapted. This oneshot was based purely off the cover art from volume 1, hence, the mangaka got to make up the entire story themselves, since they only existed as character designs before. The others would be trickier as you'd have to coordinate with the original author and such, and I'm sure there are other contractual things in the way that made this one the easiest to serialize. 😄