So, I feel like if chapter 20 had been released before 18 and 19, I wouldn't be so mad. Developing a couple to this extent and finishing their arc allows the author some leeway. If the author wants this story to be about multiple groups/couples interacting, then the events of chapter 20 creates the perfect opening to further that story line. Now that the track team couple is official, they can safely transition into supporting side characters (for Ayaka and Yurine especially) and open up a spot for a new group to come in and get some page time.
It would have made much more sense to postpone chapters 18 and 19 to be the first couple of chapters in the next volume with that new arc and new trio of characters. In its place we could have had Ayaka, Yurine, and the Senpai/Kouhai quadrant (don't remember any of their names, honestly) leading into the festival and exploring it together as a large group. Then we could end the volume with chapter 20's track team couple finale like normal.
I just feel like introducing 3 new characters so close to the end of the volume doesn't make a whole lot of narrative sense. It seems like the potential unrequited love arc is going to take a good size chunk of the next volume to fully resolve, so why not keep it all in that volume. As it stands, it feels like chapters 18 and 19 were only written to set up the existence of the upcoming festival that was used to resolve Mizuki and Moe's arc (I actually looked up their names this time. yay). It ends up feeling like padding instead of relevant material, and if we really needed padding... well... we had a whole cast of [relatively] neglected characters the author could have fallen back on...
And right there, I managed to talk myself into being mad again. I'm gunna stop.