// add this to functions.php
//register acf fields to Wordpress API
//https://support.advancedcustomfields.com/forums/topic/json-rest-api-and-acf/

function acf_to_rest_api($response, $post, $request) {
    if (!function_exists('get_fields')) return $response;

    if (isset($post)) {
        $acf = get_fields($post->id);
        $response->data['acf'] = $acf;
    }
    return $response;
}
add_filter('rest_prepare_post', 'acf_to_rest_api', 10, 3);