Webhook did not fire after subscribed WABA to application

Question

In order to connect WhatsApp business with an application, I use embedding singup flow.

All seems well - I use subscribed_apps endpoints and get a successful response:

curl --location --request POST 'https://graph.facebook.com/v18.0/[WABA_ID]/subscribed_apps' \
--header 'Authorization: Bearer [TOKEN]'

RESPONSE:

{
    "success": true
}

When I check this endpoint by 'GET' - I also see that my application has been configured correctly:

{
    "data": [
        {
            "whatsapp_business_api_data": {
                "link": "....",
                "name": "My Provider App",
                "id": "...."
            }
        }
    ]
}

When I send a message to the WhatsApp number connected to this account, the webhook does not fire (although it is well defined and properly tested).

What could be causing the problem?