At least with BEAST it would seem, where the first two chapters were in 2016-08 and 10. I can't find any social accounts, which could indicate that they just started out, but that also makes it hard to check their history.
Unfortunately, magazines are very different from books, which makes it hard to get older content. It's unlikely we can get older issues or individual chapters, so the options come down to releasing these chapters without the previous parts, or not releasing them at all.
That was a promotional chapter that came out the same month as the book, which unfortunately was the month before we started releasing Shitsurakuten. That chapter isn't included in the book (promo material rarely is), but instead there's another book-exclusive bonus chapter that was released earlier today:
Spreads are usually stand alone illustrations or work as a bonus to upcoming/older chapters. There's no Toroshio chapter that's part of this, unless that came out over 10 months ago when we started localizing magazines.
Typically an artist only have a story every 2-3 issues, but they did sort of an event for this guy with two stories in a row for his debut. That's probably why it took a while to get the third chapter out.