I’m not exactly sure by reading your description what you are trying to do.
When you add a file field there is a meta_key that corresponds to the field. When you upload a file this file is saved as a “post” of the post type “attachment” which is how WP stores all uploaded files. The value of the ACF field corresponds with the ID value of the attachment post that is created for the file.
And ACF file field cannot hold a URL and would not no what to do with a URL in this field, so it ignores the value that is in the DB if it is a URL.
Does that help at all? Can you give a better explanation of what you are trying to do?
