This appears to be expected behavior. The 'notes_next_activity_date' field for the contact record in question is blank within the HubSpot app. The webhook subscription is notifying your app that the value changed to a blank value. This likely means that there is no next scheduled activity date. This property is one that gets automatically set when certain in-app actions occur - i.e. a sales rep schedules a meeting with a contact. Once that meeting occurs and there are no future scheduled actions, the property moves back to null - which is the hook you're being notified about here.