I mean, she's there to represent his past. There's actual meaning to her introduction and inclusion, because it's what frames the ML and his progression as a character through the course of the narrative.
I get that the "losing heroine" trope is a well-established one in romance manga, but there is merit to engaging with each interation and instance on its own merit within a given series, and not let preconceived notions or biases color or limit your analysis and interpretation of what the character represents to the story they're in.
Also I don't think its even established that she really likes him romantically. Given her beaming with excitement when he said she is an important friend to him. Makes me think she was more worried about him not needing her anymore, and less about him possibly liking someone else. Yeah she blushes, but noone is going to smile like that, and immediately be way higher spirits after getting "Friend zoned" by the person they like. I think people here are just too romcom brained, and think everyone introduced that is the opposite sex of the protagonist HAS to be a love rival. They can't just be good friends.
What I personally think is. She was worried that with him changing so much he would stop seeing her as someone he needs and can rely on, and that scared her. She needed that reassurance that even if he changes he will still be friends with her. She mentioned the beach picture she saw. You could just as easily interpret her jealousy as not a romantic type, but more so her being jealous of the situation. Jealous that he started getting close to others, something he never did before, and her being scared that if he gets close to others he will not need her anymore.
I said this last chapter she was around. I doubt she is going to play a major role past this point. Given how she goes to a different school, and only shows up around summer time. I think this development exists, not to introduce a love rival, but to cause a sort of rift between the two main cast that will require him to come out of his comfort zone, and be more proactive. I think we will likely see fmc acting strange, and him noticing it. He will mend it in a similar way by reaffirming to fmc that she is special to him. Maybe not a confession, but a reaffirmation of the special bond he has with her.
Basically this chapter exists IMO to break the status quo he is trying to maintain, and force him to be proactive in mending this misunderstanding. This in essence is the best kind of misunderstanding.
(Gonna continue to yap)
People see a misunderstanding and immediately get up in arms about it, but not every misunderstanding is dragged out. Sometimes they are used to pull the plot from a stagnant point. To shatter the status quo. I doubt this manga is going to drag it out. I have not gotten that kind of vibe from the mangaka.