I don't think this development feels very forced or unnecessary at all. It actually makes a lot of sense to me, honestly.
This is p much their first encounter with a couple of things, namely direct opposition to their relationship and the jealousy that's brought on by said opposition knowing a side of your lover that you don't.
Mashiro was already struggling with feeling like a burden bc of how Nagisa and her met. Now someone who knows her gf from a time "before her" comes along, tells her to her face that she's a bad influence on Nagisa and she beats herself up over feeling jealous of that person. A real hattrick of insecurity.
Meanwhile Nagisa reels from being ambushed by the pressure of someone idolizing her and having to put her foot down against their talking down on her gf. And then when she catches a glimpse of Mashiro's cracking facade she gets real concerned of course, the time when her gf broke down bc she found out about their first meeting prob still fresh in her mind.
But instead of talking to her about it Mashiro tries to play it off, which hits Nagisa with a double whammy of "Oh, I guess she doesn't trust me enough to confide in me" and "What if she runs away from me again?", making her feel hurt.
And thus is the volatile cocktail of hurt feelings and uncommunicated worries created. All in all i think this is a normal, and dare I say necessary, direction for their relationship. Working through conflict makes a bond stronger after all. And the chapter is even titled "Misunderstanding", smth like this was inevitable, really.
