In WordPress ACF, filter tree of taxonomy field_type

functions.php

add_filter('acf/fields/taxonomy/wp_list_categories/name=mycat-tree1', 'mycat1_filter', 10, 2);
}

function mycat1_filter($args, $field) {
	$cat = get_category_by_slug('mycat1-slug');
	if (empty($cat)) {
		return $args;
	}
	$args['parent'] = $cat->cat_ID;
	return $args;
}