Hi @digitalquery
Good news. I have just added and pushed to github an update for the taxonomy field.
This update contains a new filter allwoing you to customize the args used in the wp_list_categories
function.
You can use it like so:
add_filter('acf/fields/taxonomy/wp_list_categories', 'my_taxonomy_args', 10, 2);
function my_taxonomy_args( $args, $field )
{
// do stuff to $args
return $args;
}