<?php

add_filter('acf/settings/load_json', 'my_acf_json_load_point');

function my_acf_json_load_point( $paths ) {

    // remove original path (optional)
    unset($paths[0]);


    // append path
    $paths[] = get_stylesheet_directory() . '/my-custom-folder';


    // return
    return $paths;

}

?>