Client.is_connected returns False even though connected

Question

I am trying to use the Python Paho MQTT library in a way that I can hook into the thread executing the client, so that I can supervise this thread and take action should it hang for some reason (watchdog).

The following code works, if I maintain my own connection state in the connected variable, but if I use client.is_connected() instead, it doesn't work, because is_connected returns false, even if I can see on the network that the connection has been established.

import paho.mqtt.client as mqtt

TOPIC = 'test156751'


def on_message(client: mqtt.Client,
               user_data: any,
               message: str):
    message = message.payload.decode()
    print(f'Received message: {message}')


client = mqtt.Client()
client.on_message = on_message

client.connect_async('test.mosquitto.org', 1883, 60)
connected = False

while True:
    # Add watchdog here
    if connected:  # client.is_connected():
        client.loop_read()
        client.loop_write()
        client.loop_misc()
    else:
        print('Connecting')
        client.reconnect()
        print('Probably connected')
        connected = True
        client.subscribe(TOPIC)
        client.publish(TOPIC, 'test message')

What am I doing wrong?