Integration with Twilio+Whatsapp successful but response from webhook not reaching WhatsApp

I've completed the integration with Twilio (for WhatsApp) as per the instructions and I'm able to get responses from my dialogflow agent for any intent that does not have webhook fulfillment (in my case it's a node.js app).

Any intent that has a webhook fulfillment, it sends only the default text response from the agent, even though the node.js webhook is receiving the request and sends a successful response, I'm getting only the default response from agent in WhatsApp through Twilio.

Can someone help on this?