Are you referring to not receiving webhook notifications for contact creation events?
If so, I see your app (ID ending in 1886) is not connected to your production HubSpot account (Hub ID ending in 8713). In order to listen for contact creation events, the app must be connected via OAuth.
The Get recently updated and created contacts endpoint will return records which have been modified, but you are correct that neither of these endpoints returns only the modified data.
If you only want to receive notifications for changes, the Webhooks API will probably be your best bet. You can subscribe to specific object properties and HubSpot will notify a webhook URL of your choosing whenever an object's value changes.
If so, you can create webhook subscriptions to custom properties, but you have to manually enter the internal name and click Create option "[internal property name]".
Are you referring to not receiving webhook notifications for contact creation events?
If so, I see your app (ID ending in 1886) is not connected to your production HubSpot account (Hub ID ending in 8713). In order to listen for contact creation events, the app must be connected via OAuth.