Assuming you’re using this in the ‘register_field_group()’ function? What is your field group setting(s)? Is the data saving to the database correctly?
EDIT: I think I read your question wrong (or too quickly).
ACF won’t automatically recognize a custom save_format – you’ll have to add this in yourself via a custom field type. I would take a look at the textarea field class and modify the ‘format_value_for_api()’ method (in your custom field type class).
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