It could be that the lighting conditions are not good. For example, if it's too dim, she might need to squint to read the fanfiction book clearly. Also, if the font size is too small, that can cause her to squint as well.
Maybe Potter has been hit by a spell that's affecting his vision and he's squinting to try and see Draco clearly. It could be part of a battle scene where Draco is an unexpected figure in the chaos.
A simple plot could be that the girl is a bookworm. One day, she forgets her glasses at home while at school. She has to squint during a reading session in the library. As she squints, she starts to imagine the characters in the book coming to life in a different way than if she had her glasses on. This could lead to a unique adventure where the blurry vision creates a whole new world within the story she's reading.
Another reason could be that the font size of the fanfiction she is reading is too small. Some websites or e - books might have a default small font, and rather than adjusting the settings right away, she just squints to be able to read the text for the moment.
Another reason might be related to a new magical artifact or potion. Harry might have come across something that had an unforeseen effect on his glasses. It could be that a new charmed item he was using interacted with his glasses and caused them to disappear.
Maybe Dumbledore wanted to protect Harry from something specific. By changing his gender, he could hide Harry from certain threats. It could also be a way to teach Harry something, like seeing the wizarding world from a different perspective.
One possible reason could be for a mission. Maybe in the fanfiction, there is a mission that requires Naruto to go undercover as a girl. This would allow him to access certain places or information that would be otherwise unavailable to him as a boy. Another reason might be for self - discovery. Naruto could be curious about what it's like to be a girl and decides to explore that side of himself in the fictional world.
One reason could be a devil fruit power gone wrong. Maybe he ate a fruit that was supposed to give him some new ability but had an unexpected side effect of changing his appearance to that of a girl. Another possibility is that he was hit by some strange magic or curse during an adventure on an island. It could also be the result of a prank by some powerful and mischievous character who has the ability to change appearances.
One reason could be as a form of punishment. Maybe Percy did something that Artemis considered disrespectful to the female gender or to her values as a goddess. Another reason might be for a mission. Artemis could think that in a female form, Percy would be able to infiltrate a place or situation that is only accessible to females.
One possible reason could be a powerful curse placed on her. In the world of Lost Girl, there are many dark forces and magic users who might want to harm Bo. Maybe she got on the wrong side of a powerful Fae who has the ability to cast deadly curses.
In the fanfiction, Harry squinting at Wormtail might also be a moment of intense focus. Harry could be trying to use his Legilimency skills (if he has developed them in the fanfic world) to read Wormtail's mind. He's squinting as he concentrates, trying to break through any mental defenses Wormtail might have. It could be a crucial moment in the story where Harry's discovery of Wormtail's thoughts could change the course of events.