Oh, i was under the assumption that you are using the acf frontend form. My bad.
The hook ‘prepare_field’ only get called on the acf’s rendered form.
But if you are rendering the form yourself, you can try ‘load_field’ hook instead.
Like this: acf/load_field/key=field_59ae96d970a06
https://www.advancedcustomfields.com/resources/acf-load_field/
Cheers
