Fluentd Forwarder pods getting flooded with warning messages when used in "Forwarder-Aggregator" mode.

Describe the bug

I am using Forwarder-Aggregator architecture of Fluentd to send my application logs to Splunk with Fluentd running in both Forwarder and Aggregator. Forwarder --> Forwarder deployed as Daemonset which collects logs from node and send it to Aggregator. Aggregator --> Aggregator deployed as Deployment and collects logs from Forwarder to send it to Splunk.

When Aggregator shuts down because of some reasons, I start getting below warnings message in Forwarder and then it never stops unless and until we don't kill forwarder pod.

2023-02-15 13:57:29 +0000 [warn]: #0 ack in response and chunk id in sent data are different chunk_id="5f4bd754fe2XXXXXX3a872fa077f94" ack="X0vXJzXXXXXXXXHR243iw==\n"

Fluentd Version : fluent/fluentd:v1.15.1-debian-1.1

Grafana graphs shows both Forwarder and Aggregator has enough CPU and Memory when this warning messages starts generating in Forwarder.

One more point I would like to add is, we see such warnings only if we set "flush_thread_count" value other than 1 in below give forward block.

To Reproduce

Run Fluentd in Forwarder-Aggregator mode and then shut down Aggregator

Expected behavior

Forwarder should stop generating mentioned warning messages when Aggregator recovered from shutdown.

Your Environment

- Fluentd version:  fluent/fluentd:v1.15.1-debian-1.1
- TD Agent version: 
- Operating system:
- Kernel version:

Your Configuration

Worker-0 of Forwarder:

<match **>
        @type forward
        send_timeout 60s
        recover_wait 10s
        hard_timeout 60s
        require_ack_response true
        heartbeat_type tcp
        keepalive true
        keepalive_timeout 30s
        <service_discovery>
          @type srv
          service tcp
          proto tcp
          interval 20
          weight 60
          hostname fluentd-aggregator-a
        </service_discovery>
        <service_discovery>
          @type srv
          service tcp
          proto tcp
          interval 20
          hostname fluentd-aggregator-b
          weight 60
        </service_discovery>
        <buffer>
          @type file
          total_limit_size 5GB
          path /var/fluentd_out/buffer_1
          queue_limit_length 100000
          chunk_limit_size 30000000
          overflow_action drop_oldest_chunk
          chunk_full_threshold 0.008
          flush_interval 10s
          flush_thread_count 8
          retry_forever false
          retry_timeout 1h
        </buffer>
        <secondary>
          @type file
          path /var/fluentd_out/out_plugin_secondary_1
        </secondary>
      </match>

Your Error Log

2023-02-15 13:57:29 +0000 [warn]: #0 ack in response and chunk id in sent data are different chunk_id="5f4bd754fe2XXXXXX3a872fa077f94" ack="X0vXJzXXXXXXXXHR243iw==\n"

Additional context

No response