num_rows() not working with sqlsrv driver and MSSQL

I'm getting the following error on a simple query that's supposed to return a number of rows:

Type: Error
Message: Call to a member function num_rows() on boolean
Filename: /var/www/dev/docroot/project/system/database/DB_query_builder.php
Line Number: 1428

I'm using Codeigniter 3.1.9. I recently started using PHP7.2 and also sqlsrv driver to connect to a MSSQL database.
In some other post, someone mentioned that scrollable should be set to an option different than SQLSRV_CURSOR_FORWARD so I dump the value of $this->scrollable in sqlsrv_driver.php and found out that its value is buffered

Code:

/**
    * Execute the query
    *
    * @param   string  $sql    an SQL query
    * @return  resource
    */
   protected function _execute($sql)
   {
       echo $this->scrollable; die();
       return ($this->scrollable === FALSE OR $this->is_write_type($sql))
           ? sqlsrv_query($this->conn_id, $sql)
           : sqlsrv_query($this->conn_id, $sql, NULL, array('Scrollable' => $this->scrollable));
   }

Not quite sure why it's failing. Any other queries that don't include num_rows() are fine so far. Thanks.