Controlling the number of processes run by the out_exec plugin

I’m using the in_tail plugin to monitor log files.
Logs read by the in_tail plugin are configured by td-agent.conf to be processed by the out_exec plugin.
I setted flush_thread_count in the buffer section because I want to limit the number of concurrent python scripts executed by out_exec.

<source>
  @type tail
  path /var/data/app/log/target.log
  pos_file /var/data/td-agent/target.log.pos
  tag target_log

  <parse>
    @type tsv
    keys message
  </parse>
</source>

<match target_log>
  @type exec
  command /bin/python3 /usr/local/scripts/main.py
  command_timeout 3600

  <format>
    @type tsv
    keys message
  </format>

  <buffer time>
    @type file
    path /var/data/buffer_target/
    flush_mode interval
    flush_interval 1s
    flush_thread_interval 5.0
    flush_thread_burst_interval 30.0
    flush_thread_count 1
    retry_wait 10s
    retry_max_interval 60s
    retry_max_times 3
    chunk_limit_size 10MB
    total_limit_size 10GB
    queued_chunks_limit_size 1
    delayed_commit_timeout 4000
    timekey 1h
  </buffer>
</match>

However, after in_tail reads consecutive log writes, chunks are enqueued at every flush_interval interval, and python scripts are run concurrently, ignoring the flush_thread_count setting.
Even if I explicitly set queued_chunks_limit_size to 1, the behavior was the same.

ls -ltr:

-rw-r--r-- 1 td-agent td-agent 842500  6月  8 15:55 buffer.q5c43ba57f0b910f253949bb8c7d32928.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:55 buffer.q5c43ba57f0b910f253949bb8c7d32928.log.meta
-rw-r--r-- 1 td-agent td-agent 842500  6月  8 15:55 buffer.q5c43ba5cb729b37e2ff23aa51743f51c.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:55 buffer.q5c43ba5cb729b37e2ff23aa51743f51c.log.meta
-rw-r--r-- 1 td-agent td-agent 842500  6月  8 15:55 buffer.q5c43ba617f55e2915b29c8e5883add75.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:55 buffer.q5c43ba617f55e2915b29c8e5883add75.log.meta
-rw-r--r-- 1 td-agent td-agent 674000  6月  8 15:56 buffer.q5c43ba66478bf35554ceed5ac7599c89.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:56 buffer.q5c43ba66478bf35554ceed5ac7599c89.log.meta
-rw-r--r-- 1 td-agent td-agent 842520  6月  8 15:56 buffer.q5c43ba6a19b33a4169e1316aca91b46a.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:56 buffer.q5c43ba6a19b33a4169e1316aca91b46a.log.meta
-rw-r--r-- 1 td-agent td-agent 842600  6月  8 15:56 buffer.q5c43ba6ee0ddbf281ad3dc3195a08a0e.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:56 buffer.q5c43ba6ee0ddbf281ad3dc3195a08a0e.log.meta
-rw-r--r-- 1 td-agent td-agent 842600  6月  8 15:56 buffer.q5c43ba73ac59eebc92cc266c9dc10a71.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:56 buffer.q5c43ba73ac59eebc92cc266c9dc10a71.log.meta
-rw-r--r-- 1 td-agent td-agent 842600  6月  8 15:56 buffer.q5c43ba7874ec3453c7be39e3f3374534.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:56 buffer.q5c43ba7874ec3453c7be39e3f3374534.log.meta
-rw-r--r-- 1 td-agent td-agent 842600  6月  8 15:56 buffer.q5c43ba7d3c88c61456a9676dc488315b.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:56 buffer.q5c43ba7d3c88c61456a9676dc488315b.log.meta
-rw-r--r-- 1 td-agent td-agent 842600  6月  8 15:56 buffer.q5c43ba82088a75471e1c35caba097820.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:56 buffer.q5c43ba82088a75471e1c35caba097820.log.meta
-rw-r--r-- 1 td-agent td-agent 842600  6月  8 15:56 buffer.q5c43ba86c9d7a769ed3268c9c99cf890.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:56 buffer.q5c43ba86c9d7a769ed3268c9c99cf890.log.meta
-rw-r--r-- 1 td-agent td-agent 842600  6月  8 15:56 buffer.q5c43ba8b90fa4592ed8232c60ba6f736.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:56 buffer.q5c43ba8b90fa4592ed8232c60ba6f736.log.meta
-rw-r--r-- 1 td-agent td-agent 842600  6月  8 15:56 buffer.q5c43ba9058fcd955b0ed8738492e955f.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:56 buffer.q5c43ba9058fcd955b0ed8738492e955f.log.meta
-rw-r--r-- 1 td-agent td-agent 842600  6月  8 15:56 buffer.q5c43ba9520b7f8be64d09c54e5b34d86.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:56 buffer.q5c43ba9520b7f8be64d09c54e5b34d86.log.meta
-rw-r--r-- 1 td-agent td-agent 842600  6月  8 15:56 buffer.q5c43ba99e92bfb20675c32ef02c54f39.log
-rw-r--r-- 1 td-agent td-agent     81  6月  8 15:56 buffer.q5c43ba99e92bfb20675c32ef02c54f39.log.meta

ps -auxwww:

td-agent  934669  8.8  1.2 125524 48096 ?        R    15:55   0:18 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba57f0b910f253949bb8c7d32928.log
td-agent  934715  8.5  1.1 121428 47800 ?        R    15:55   0:17 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba5cb729b37e2ff23aa51743f51c.log
td-agent  934767  8.1  1.1 121428 47888 ?        R    15:55   0:15 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba617f55e2915b29c8e5883add75.log
td-agent  934817  7.7  1.1 120916 47324 ?        R    15:56   0:14 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba66478bf35554ceed5ac7599c89.log
td-agent  934870  7.3  1.1 121428 47928 ?        R    15:56   0:13 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba6a19b33a4169e1316aca91b46a.log
td-agent  934913  7.1  1.1 121428 47776 ?        R    15:56   0:12 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba6ee0ddbf281ad3dc3195a08a0e.log
td-agent  934966  6.7  1.1 121428 47732 ?        R    15:56   0:11 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba73ac59eebc92cc266c9dc10a71.log
td-agent  935019  6.6  1.1 121428 47652 ?        R    15:56   0:11 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba7874ec3453c7be39e3f3374534.log
td-agent  935068  6.4  1.1 121428 47812 ?        R    15:56   0:10 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba7d3c88c61456a9676dc488315b.log
td-agent  935113  6.2  1.1 121428 47760 ?        R    15:56   0:10 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba82088a75471e1c35caba097820.log
td-agent  935166  5.9  1.1 121428 47844 ?        R    15:56   0:09 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba86c9d7a769ed3268c9c99cf890.log
td-agent  935218  5.8  1.1 121428 47624 ?        R    15:56   0:08 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba8b90fa4592ed8232c60ba6f736.log
td-agent  935261  5.7  1.1 121428 47852 ?        R    15:56   0:08 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba9058fcd955b0ed8738492e955f.log
td-agent  935313  5.5  1.1 121428 47616 ?        R    15:56   0:07 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba9520b7f8be64d09c54e5b34d86.log
td-agent  935365  5.5  1.1 121428 47636 ?        R    15:56   0:07 /bin/python3 /usr/local/scripts/main.py /var/data/td-agent/buffer_target/buffer.q5c43ba99e92bfb20675c32ef02c54f39.log

Also, in version 0.12 of td-agent, it was possible to control the number of concurrent executions of Python scripts with a similar setting.
How can I control the number of concurrent executions of python scripts with version 1.0 of td-agent?