Spring Kafka Producer blocking http thread till Kafka thread complete

We have built an application using spring boot for enterprise reasons and using the spring Kafka project to publish Kafka messages.

I have been testing producing Kafka events and noticed that spring is blocking the HTTP thread till the Kafka thread is complete.

try {
ListenableFuture<SendResult<String, OnboardingSigningNotificationEvent>> send = this.kafkaTemplate.send(TOPIC, notificationEvent);

  send.addCallback(new ListenableFutureCallback<SendResult<String, OnboardingSigningNotificationEvent>>() {
    @Override
    public void onSuccess(SendResult<String, OnboardingSigningNotificationEvent> sendResult) {
      log.info("Successfully published onboarding signing notification event with record metadata timestamp: {}", sendResult.getRecordMetadata().timestamp());
    }
    @Override
    public void onFailure(Throwable throwable) {
      log.error("Failure while publishing kafka event : {}", throwable.getMessage());
    }
  });
} finally {
  kafkaTemplate.flush();
}

This contradicts what is said in this documentation. https://docs.spring.io/spring-kafka/docs/current/reference/html/#sending-messages

Notice that the send methods return a ListenableFuture. You can register a callback with the listener to receive the result of the send asynchronously.

Based on the documentation I would expect the producer call to be made asynchronously and the main http thread to not block waiting for it, but that's not what I see in my tests. Even if I block the kafka thread for a few seconds the caller http thread is being blocked.

Can you please explain whether it is on purpose and are there any config to override this behavior.