I guess because the previous issue of Galette announced this as being the last chapter, the next chapter couldn't simply be chapter 21?
Aikata System is a series by Hakamada Mera that was original running for label Lilie comics, but that label died with Aikata System unfinished at chapter 17. So the series got revived in crowdfunded yuri magazine Galette, starting over from chapter 1, and contributing quite a large portion of the page count of some Galette issues. Once it got to the new chapters, there's a literally random, unpleasant incident that has dominated the conclusion to the story. It's got so little to do with what the story had been about that it makes me suspect Hakamada just wanted to be done with the series. But she hasn't quite managed to get it done yet.