Every 8 minutes a new thread is added in the pool

[pabasaraCS]

Hello celery team,
We run celery 4.1.1, django celery 3.2.2, with Python 3.5.5, ampq 2.3.2, mysqlclient 1.3.12.
We have a job defined in mysql:

djcelery_periodictask={"id":"5","name":"quote","task":"quote","args":"[]","kwargs":"{}","queue":null,"exchange":null,"routing_key":null,"expires":null,"enabled":"1","last_run_at":"2019-03-27 13:52:00.001668","total_run_count":"147","date_changed":"2019-03-27 13:52:15.029632","description":"","crontab_id":"5","interval_id":null}
with :
djcelery_crontabschedule = {"id":"5","minute":"*\/2","hour":"9-22","day_of_week":"mon-fri","day_of_month":"*","month_of_year":"*"}

we use rabbitmq.

We save manage the time is UTC, the server is in EST.

The job runs every 2 minutes, but we notice that every 8 minute the job adds a new thread in the pool. For example, at the beginning, we have 1 thread running in the pool, the 8 minutes later, 2 threads are running every 2 minutes, then 8 minutes later, 3 threads are running every 2 minutes, and so forth.

...
Do you know how we can investigate this issue?

Thank you very much
Best regards
Pabasara Ranathunga