The add_meta_boxes function in admin-filters-post.php presently uses the following logic to determine whether or not to replace the meta box:
if ($taxonomy->show_ui && !is_taxonomy_hierarchical($tax_name))If you consult the documentation for the meta_box_cb option to register_taxonomy (notably "No meta box is shown if set to false."), it is apparent that this should probably be:
$customMeta = isset($taxonomy->meta_box_cb) && is_callable($taxonomy->meta_box_cb);
if ($taxonomy->show_ui && !$customMeta && !is_taxonomy_hierarchical($tax_name))Default behavior for a taxonomy with show_ui = true, meta_box_cb = false is a taxonomy with taxonomy management pages but no meta boxes.
