The bug causing the auth failures is here:

            ResponseStatus status(exceptionToStatus());
            asio::post(_io_service, [this, op, status]() { return _completeOperation(op); });
            return;

We should set the ResponseStatus constructed from the exception as the result of executing the command, but we don't. As a result, we don't hit special logic in TopologyCoordinatorImpl::processHeartbeatResponse that is used for handling authentication failures in heartbeats.