Django Celery Periodic task is not running on given crontab

Question

I am using the below packages.

celery==5.1.2
Django==3.1

I have 2 periodic celery tasks, in which I want the first task to run every 15 mins and the second to run every 20 mins. But the problem is that the first task is running on time, while the second is running on random timing.

Although I'm getting a message on console on time for both tasks:

Scheduler: Sending due task <task_name> (<task_name>)

Please find the following files,

celery.py

from celery import Celery, Task
app = Celery('settings')
...

class PeriodicTask(Task):
    @classmethod
    def on_bound(cls, app):
        app.conf.beat_schedule[cls.name] = {
            "schedule": cls.run_every,
            "task": cls.name,
            "args": cls.args if hasattr(cls, "args") else (),
            "kwargs": cls.kwargs if hasattr(cls, "kwargs") else {},
            "options": cls.options if hasattr(cls, "options") else {}
        }

tasks.py

from celery.schedules import crontab
from settings.celery import app, PeriodicTask
...

@app.task(
    base=PeriodicTask,
    run_every=crontab(minute='*/15'),
    name='task1',
    options={'queue': 'queue_name'}
)
def task1():
    logger.info("task1 called")

@app.task(
    base=PeriodicTask,
    run_every=crontab(minute='*/20'),
    name='task2'
)
def task2():
    logger.info("task2 called")

Please help me to find the bug here. Thanks!