db_select's addExpression does not honor order in which it was added

With Drupal 7 (HEAD), I am using the TableSort extender in a custom module to output some data in a sortable table. The problem is that if I add an expression (with $query->addExpression()) the query does not respect the order in which I added the expression. If I have 3 columns (1 real column, 1 expression, and another real column), it always puts the expression last, under the wrong heading in the table.

No matter what order expressions and fields are added to a select query, the expressions always come after all the fields. Am I doing something wrong, or should I be filing a bug report?

Here's the code:

  // $headers for theme_table and TableSort extender
  $headers = array(
    array( 'data' => 'Manufacturer', 'field' => 'company_name', 'sort' => 'asc' ),  // FIRST column
    array( 'data' => 'Item', 'field' => 'itemlink'),  // SECOND column
    array( 'data' => 'Abbr.', 'field' => 'abbreviation')  // THIRD column
  );

  // Set up the query
  $query = db_select('battery_model_list_view');
  $query = $query->extend('TableSort');
  $query = $query->orderByHeader($headers);  
  $query->addField('battery_model_list_view', 'company_name');  // FIRST db column
  $query->addExpression("'<a href=\"/batt/details/' || batt_model_id ||'\">' || model || '</a>'", 'itemlink');  // SECOND db column
  $query->addField('battery_model_list_view', 'abbreviation');  // THIRD db column

  // Execute the query
  $result = $query->execute();
  $rows = $result->fetchAll(PDO::FETCH_ASSOC);
  
 // Create $table variables for theme_table()
  $table = array(
    'header' => $headers,
    'rows' => $rows,
    'caption' => null,
    'attributes' => array(),
    'colgroups' => array(),
    'sticky' => null,
    'empty' => null,
  );

   $out = "<p>The following is a list of items.  Click the model to see details of that model</p>";
  
  $out .= theme_table($table);
  
  return $out;