letters!!

Hi;
My name is Thelma and I have a question about the endings of formal letters.
I know that we should end with "Yours sincerely or Yours Faithfully" but in the last movie of Harry Potter, when HP receives a letter it says at the end "HOPING YOU ARE WELL" is that expression incorrect?
Thank you for your attention and I will wait for your reply.
Thelma

[Snowcake]

Yours sincerely
The writer knows the recipient's name. (Dear Mr Smith / Dear Mrs McKenzie)

Yours faithfully
The writer does not know the recipient's name (Dear Sir / Madam)

HOPING YOU ARE WELL" is that expression incorrect?

It's not incorrect, but not appropriate in formal letters.