Session message disappears if db transaction is enabled - bug?

I use flash messages to display simple error messages to the user in the event of database error. The full error stack trace gets written to the log. I have found a repeatable issue in which the flash message (and non-flash as well) disappears if the db error is thrown inside of a db transaction.

In my form controller I catch any exception thrown within my model ($this->donation->update() is a library method, which calls a donations_model method)
I write the error to the log (always works) and set a flash message
Then I redirect to another (ledger) controller, which will display the message

Code:

            try {
                $this->donation->update($transaction_id, $program_id, $friend_id, $gross, $fee, $agency_transaction,
                                        $date, $receipt_number, $donation_revenue, $donation_asset);
            } catch (Exception $e) {
                //logging works with or without db transaction
                log_message('error', $e->getMessage());
                //This flash message only remains if db transaction is commented out
                $_SESSION['message'] = 'There was an error updating the donation record';
                $this->session->mark_as_flash('message');
                //The session message can always be echoed here with or without a db transaction
                echo $_SESSION['message'];
            }
            redirect('ledger');

The ledger controller checks for any flash messages. If none, then it sets the message in the view to ''

Code:

//The message is undefined if the error in the model is thrown within a transaction.
        // But It prints successfully if the transaction is commented out in the model
        echo $_SESSION['message'];
        if (isset($_SESSION['message'])) {
            $view_content['message'] = $_SESSION['message'];
        }
        else {
            $view_content['message'] = '';
        }

Here is the method in the donations_model with a deliberate SQL error (look for 'Hey man!') and a thrown exception
If the transaction is commented out then the flash message will successfully remain in the session and can be displayed on the ledger page

Code:

    public function update($transaction_id, $friend_id, $gross, $fee,
                            $net, $agency_transaction, $receipt_number) {
        // Session message remains set if the transaction is commented out
        //$this->db->trans_start();
        $this->db->where('transaction_id', $transaction_id);
        $data = array(
                'friend_id'        => 'Hey man!', //$friend_id,
                'gross'            => $gross,
                'fee'            => $fee,
                'net'            => $net,
                'agency_transaction' => $agency_transaction,
                'receipt_number'    => $receipt_number
        );
        if (!$this->db->update('donations', $data)) {
            $error = $this->db->error();
            throw new Exception('Donations_model->update: ' . $error['code'] . ' ' . $error['message']);
        }
        //$this->db->trans_complete();
        return TRUE;
    }

If you need more details for a bug report I can provide a link to my project on github.