The core behavior for a user-managed app is that Zoom uses OAuth to subscribe to webhook events for the user that granted these permissions, so group membership alone won’t generate user webhooks unless those users are actually added/authorized
If what you did was just approval, note that pre-approve the app for certain users or user groups is different from admin provisioning, where admins can add the app for users… as authorization has already been done by the admin
To verify who Zoom thinks is provisioned, follow the admin steps to open Manage… Apps on Account and confirm the list of current users with the app is displayed; if the expected users aren’t listed, re-add them via Add for Others