Subscribe callback not working with connect_async in paho-mqtt python

Question

I've been trying to use the connect_async() method of paho-mqtt Client class. The on_connect() callback is working fine but the on_subscribe() callback is not working. How could I make it work?

import paho.mqtt.client as paho
from paho import mqtt
import time

def on_connect(client, userdata, flags, rc, properties=None):
    print("On connect called!!")

def callback(client, userdata, message):
    print(str(message.payload.decode("utf-8")))

def on_subscribe(client, userdata, mid, granted_ops, properties=None):
    print("On subscribe called!!")

client_get = paho.Client(client_id='my_client', protocol=paho.MQTTv5)
client_get.tls_set(tls_version=mqtt.client.ssl.PROTOCOL_TLS)
client_get.on_connect = on_connect
client_get.username_pw_set("USERNAME", "PASSWORD")
client_get.connect_async('HOST_URL', 8883)

client_get.on_message = callback
client_get.on_subscribe = on_subscribe
client_get.loop_start()
client_get.subscribe("test/#", qos=1)

while True:
    print("1")
    time.sleep(10)