The datastore client's RunInTransaction() contains this piece of code that seems to pass down the previous transaction id when retrying a transaction because it previously conflicted with another transaction:
https://github.com/GoogleCloudPlatform/google-cloud-go/blob/8c9fc24c2b85cbc71e8f1dbb6b7ac70a20bfe1c2/datastore/transaction.go#L176-L180

However, just before this code we call Transaction.Commit() and after Commit(), the transaction id is always nil. I'm not sure what the backend does with this data, but this code should either be fixed or removed.