phpbb\db\driver\driver is not able to build subqueries. The following code

$sql = $this->db->sql_build_query('SELECT', array(
     'SELECT'    => 'group_id,user_id',
     'FROM'        => array(USER_GROUP_TABLE => 'ug'),
     'WHERE'        => array('AND', array(
         array('user_id', 'NOT IN', '', 'SELECT', array(
             'SELECT'    => 'user_id',
             'FROM'        => array(USERS_TABLE => 'u'),
         )),
     )),
     'ORDER_BY'    => 'user_id,group_id',
 ));

gives the following statement:

SELECT group_id,user_id FROM (forum1_user_group ug) WHERE user_id NOT IN SELECT ( ) ORDER BY user_id,group_id

The Sub-Select-Statement is missing.

(The code above is just a simplified / changed exmaple for easier testing.)

The reason for that is driver::_process_boolean_tree :

      $condition = $condition[self::LEFT_STMT] . ' ' . $condition[self::COMPARE_OP] . ' ' . $condition[self::SUBQUERY_OP] . ' ( ';
      $condition .= $this->sql_build_query($condition[self::SUBQUERY_SELECT_TYPE], $condition[self::SUBQUERY_BUILD]);
      $condition .= ' )';

The first line modifies the condition-array to a string. When the second line tries to read array values, it instead access specific characters of the string that the first line wrote to $condition.