I found this, looks like what you’re looking for, https://generatewp.com/add-custom-fields-to-featured-image-meta-box/. You’ll need to use the ACF field names where field names are used in the example. Also, you will need to update the field keys. Or, alternately, you might be able to use update_field() instead of update_post_meta(), just make sure you use the field keys when updating ACF field values https://www.advancedcustomfields.com/resources/update_field/
