Returning from a gateway in a subflow using java config when using a replyChannel

Question

I have the following configs

@Bean
public IntegrationFlow mainFlow(){
 return IntegrationFlows.from("channel1")
        .route("condition", p -> p
                .subFlowMapping("true", flow -> flow.gateway("channel2"))
                .subFlowMapping("false" flow -> {
                    flow.gateway("channel3", c -> c.replyChannel("aggregatorOutputChannel"));
                    flow.gateway("channel4");
                 })
        .get();
}

@Bean
public MessageChannel channel3()
{
    return MessageChannels.publishSubscribe().applySequence(true).get();
}

@Bean
public IntegrationFlow flow1(){
  return IntegrationFlows.from("channel3")
     .transform(s -> {
        System.out.println(s);
        return s;
      }
     .channel("aggregatorInputChannel")
     .get();
}

@Bean
public IntegrationFlow flow2(){
  return IntegrationFlows.from("channel3")
     .split()
     .transform(s -> {
        System.out.println(s);
        return s;
      }
     .aggregate()
     .channel("aggregatorInputChannel")
     .get();
}

@Bean
public IntegrationFlow aggregatorFlow(){
        return IntegrationFlows.from("aggregatorInputChannel")
                .aggregate((AggregatorSpec s) -> s.groupTimeout(60000)
                        .outputProcessor(new AggregationMessageGroupProcessor())
                        .sendPartialResultOnExpiry(true)
                        .expireGroupsUponCompletion(true)
                        .releaseStrategy(new TimeoutOrSequenceCountComparatorReleaseStrategy(4, 60000)), null)
                .gateway("postAggregation")
                .channel("aggregatorOutputChannel")
                .get();
}

Channel 3 is a pub-sub channel and there are 2 consumers that process the message and put a message in the aggregatorInputChannel after they are done. However what I am observing is that even though the message is finally put in the aggregatorOutputChannel (I debugged though the aggregator bean definition and I can see the message going into the aggregatorOutputChannel), the gateway on channel3 blocks and never returns. As a result the message never goes to channel4.

Am I missing something here? Or doing something wrong?

AggregationMessageGroupProcessor is my custom aggregator in which all I do is override the aggregatePayloads method

public class AggregationMessageGroupProcessor extends AbstractAggregatingMessageGroupProcessor {

  @Override
  protected final Object aggregatePayloads(MessageGroup group, Map<String, Object> headers) {
      group.getOne().getPayload();
  }
}

Thanks!