webhook not receiving events from Messenger recurring message opt-in

Question

Facebook recently released an optional Recurring Notifications feature for Messenger.

My group has attempted to use this feature in our app; in our chatbot flow, we present the user with the opt-in message, and when they click on the opt-in button, we are supposed to get a webhook event notification. Unfortunately, no request is coming to our webhook when the user clicks the button.

We've done some sanity checks:

• The app is subscribed to messaging_optins notifications feed (v11).
• I'm certain the webhook itself is configured properly, because our server is receiving other notification feeds (e.g. messages).
• The problem also doesn't seem to be with our opt-in request button configuration, because even the Test button in the Facebook Developers dashboard (which is supposed to send a sample messaging_optins payload to our server) does not result in a call to our webhook.

Possibly irrelevant details: the page connected to this app is registered in Kenya and the webhook server is running on an AWS instance in Germany. We have another page registered in India connected to AWS in India, and that connection is working fine despite identical configuration.

What other things should we troubleshoot to identify the problem here?