There's 2 possibilities of why manbagi gave that seat to only tadano and komi
1. This is just what they usually do manbagi gave komi-san a chance to get close to tadano and then later on komi-san would also give a chance to manbagi to get close to tadano, it's just like in the previous chapter
2. Manbagi has give up on tadano because tadano told her that he just consider manbagi as a friend on chapter 255
Or it could be something else we'll just see on the next chapter