It is an all or nothing thing, run on the entire $_POST[‘acf’] array before acf/save_post is run.
To allow it on just one field you would need to allow it on all fields and then you’d need to create your own filters for every other field except the one you want to allow. You’d have to hook in and run your own filter on $_POST[‘acf’] before ACF does… I’m not exactly sure how you’d do that. Possibly on the “init” hook with a low priority.
