get_stylesheet_directory() point to theresources/ folder

/**
 * ACF save json folder
 */
add_filter( 'acf/settings/save_json', 'my_acf_json_save_point' );
function my_acf_json_save_point( $path ) {

    return get_stylesheet_directory() . '/acf-json-custom-folder';
}

/**
 * ACF load json folder
 */
add_filter( 'acf/settings/load_json', 'my_acf_json_load_point' );
function my_acf_json_load_point( $paths ) {
    $paths[] = get_stylesheet_directory() . '/acf-json-custom-folder';

    return $paths;
}