Not receiving webhooks for NCW transaction signing

Hi all,

I’m running the frontend and backend demo apps and all works well except for signing transactions. I configured a webhook in the FB portal (see image below), exposed it to the internet (using dloudflared and a proper TLS cert) but it never gets invoked.
Webhook:

I created a few transactions of type message a in the demo app and I can see the transactions are pending signature, but like I said nothing gets invoked by FB

Eventually the transactions will timeout and fail after 2 hours

Any ideas? I only have access to sandbox env at this stage

Cheers!