You’re getting a tough time here OP.
It’s no surprise you’ve dug your heels in - your daughter told you - who picked the name - that you are pronouncing this name wrong! How ignorant, stupid and disrespectful.
It sounds like she’s trying to fully identify with her dad’s heritage and country and wants to sound more exotic than plain old Bella, or Issy. It’s pretentious and she’ll probably be embarrassed when she’s older.
Claiming that no one has called her anything except Isa is a lie. None of her UK friends or teachers would have called her that naturally unless she attempted to enforce it, which would have met with limited success. As an adult, every time she gets a non-social phone call, or called into an appointment they will call her Isabella with the English pronunciation.
A child changing their name is a rejection of their parents’ choice for them, so it will sting. It is also asking something of you that will never feel right or natural. Ultimately, it’s their choice, but she has not been respectful about it at all.
It’s not like she said, “Mum I prefer Isa and it suits me much better. This is what feels right for me and what everyone else is calling me now.” She’s stupidly said you are wrong about your own baby name choice which is incredibly rude, she is blatantly lying/exaggerating about “no one” else calls her that, and it seems like this new insistence is to make herself seem more exotic which is juvenile and may not last. And the short name she has picked is not sustainable in the UK, she will have to keep enforcing it. None of it is sensible or authentic.
I suspect she’s pretending that Eesabella is her “real name” (as opposed to different pronunciation of the same) and doesn’t want you to embarrass her by letting it slip that she’s just regular Isabella to anyone English.