No expand icons in manage groups screen

The manage groups screen lost the expand icons.

I can see the reason but I don't understand it fully. In the datatable's drawCallback in templates/CRM/Group/Form/Search.tpl table rows with class crm-group-parent are selected to prepend the expand icon. The datatables AJAX call leads to getGroupListSelector in CRM/Contact/BAO/Group.php which removes the class if only parents are requested:

if (!empty($params['parentsOnly'])) {
foreach ($value['class'] as $id => $class) {
if ($class == 'crm-group-parent')

}
}

Is this a misunderstanding of parentsOnly? Any ideas to fix?