Before forcing a write command to go through the router, the replica set endpoint does a pre-check to ensure that the node is a primary. This check is performed "too early", i.e. not inside the ServiceEntryPoint which has the code to attach the error labels (e.g. here and here). Not returning the "TransientTransactionError" label means that the drivers wouldn't retry the transaction on behalf of the user. So this needs to be fixed. 

NB: This pre-check also affects error labeling in general (e.g. "RetryableWriteError").