Maybe he wants to keep it safe from thieves. There could be dark wizards or even some of his schoolmates who might be tempted to steal it.
In fanfiction, Harry might hide his money because he's planning something big. He could be secretly funding a resistance movement against the dark forces. Or perhaps he doesn't trust the wizarding banks completely. After all, during the war, the banks could be infiltrated or controlled by the bad guys. So, hiding his money gives him more control and security over his finances.
In some fanfictions, Harry might hide his money in a secret compartment of his trunk. He could use magic to create a hidden space where no one else can find it.
He could be disillusioned with the wizarding world's politics and power struggles. So, taking his money and leaving is his way of getting away from it all. He might feel that he can't make a real difference within the existing system and decides to find his own path.
He might be holding his diaphragm because it's a part of a new magic technique he's learning. Maybe it's something taught by a mysterious new teacher at Hogwarts who believes that the diaphragm is a key element in controlling a certain type of magic energy.
Hermione could be the one. With her vast knowledge of the wizarding laws and her sharp intellect, she might dig up some old regulations that prove Harry's right to his money. She would then fight tooth and nail in the wizarding courts or against the goblins at Gringotts to get his money back.
He could use magic items like the invisibility cloak all the time. In fanfiction, the cloak might be enhanced to not only make him invisible but also mask his magical signature, so no one can detect his presence through spells.
Maybe for the sake of creating drama. Fanfic writers like to add twists to the original story. Blocking Harry's magic can create a lot of tension and make readers wonder how he'll get out of the situation.
Perhaps they fear that fanfiction could deviate too much from the original themes and values of the Harry Potter series. The official story has a certain moral and ethical framework, and fanfiction might distort it.
He might flee because fanfiction can sometimes distort his character. In some fan - made stories, his personality, values, or relationships could be changed in ways that are untrue to the original. For instance, if a fanfiction makes him act cruelly when he's supposed to be kind - hearted.
It could be that the Harry Potter brand wants to control the narrative more tightly. Fanfiction can sometimes take the story in directions that the official creators don't like. So, to maintain a certain image and story integrity, they might be discouraging fanfiction.
One reason is copyright infringement. If the fanfiction uses too much of the original Harry Potter material without permission, it can be considered forbidden. Another reason could be that it contains disturbing or immoral content that doesn't align with the family - friendly nature of the original series.