Graph API Contact Subscription Not Working

Question

I'm currently having trouble receiving any Microsoft Graph API subscription notification for Contact changes. (created, updated, deleted) I successfully create a subscription, get back a 201 response, with the subscription id, I see the subscription when calling the /v1.0/subscriptions endpoint. I see the validationToken being sent, and I pass back a 200 response as instructed by the API docs.

However, I am not seeing any calls no matter what I do with my contacts. How would I go about figuring out why that is? I can't be any firewall issues b/c we don't have any rules blocking it, especially since I see the validationToken request checking the notificationURL.