Please try doing this
add_filter('acf/settings/load_json', 'my_acf_json_load_point');
function my_acf_json_load_point( $paths ) {
// output paths and exit
print_r($paths); die;
error_log('hey!');
// remove original path (optional)
unset($paths[0]);
// append path
$paths[] = get_template_directory() . '/custom-fields';
// return
return $paths;
}