Currently, the Go driver doesn't add the TransientTransactionError label to server selection errors that happen during a transaction, despite the transactions spec requiring it. That is currently complex to implement, but may be significantly simpler after GODRIVER-2579 is done.

Original comment from divjot.arora@mongodb.com:
Hey team, per the transactions spec, server selection errors should be marked with the TransientTransactionError label:

Any command error that includes the "TransientTransactionError" error label in the "errorLabels" field. Any network error or server selection error encountered running any command besides commitTransaction in a transaction. In the case of command errors, the server adds the label; in the case of network errors or server selection errors where the client receives no server reply, the client adds the label.