I've been playing around with the Management Activity API in Office 365 as outlined in the api documentation.
Everything is working as expected, except that when I add a subscription with a notification webhook, the webhook is never called.
I set up the webhook as an Azure function, and when registering the subscription, the validation call is made to the function as expected, and I can confirm that new content is available in the management api as calls to the subscriptions/content endpoint are successful.
A call to the /subscriptions/list endpoint confirms my webhooks are registered and enabled, but a call to /subscriptions/notifications always returns an empty array.
My tenant is a Developer tenant, not sure if that makes a difference?