Details: How is \Magento\Sales\Model\Order\Payment::transactionAdditionalInfo declared and used?
\Magento\Sales\Model\Order\Payment::transactionAdditionalInfo
https://github.com/magento/magento2/blob/ffea3cd/app/code/Magento/Paypal/Model/PayflowExpress.php#L149-L161
https://github.com/magento/magento2/blob/ffea3cd/app/code/Magento/Paypal/Model/Payflow/Pro.php#L81-L100
https://github.com/magento/magento2/blob/ffea3cd/app/code/Magento/Paypal/Model/Payflow/Pro.php#L126-L140
See also:
\Magento\Sales\Model\Order\Payment::setTransactionId()