When ACF calls your filter it sends all of the field group information. You need to alter that information. This means that you need to add values to that nested array.
add_filter(‘acf/get_field_group’, ‘my_modify_field_group_function’);
function my_modify_field_group_function($group) {
if ($field[‘key’] != ‘group_58c6bfd3a0239’) {
// not our field group
return;
}
// add an OR rule to existing location rules for a specific field group
$group[‘location’][] = array(
array(
// your rule here
),
);
return $group;
}