The subscr_failed event with a Denied status is meant to log a failed payment row, but isn't currently being handled in the notification handling. Will review implementing handling of those events in the notification behavior, but it doesn't do anything except log a failed payment row.
forge.joomlapolis.com/issues/7678
For now I suggest just enabling the notifications at Stripe, which you can do under Settings > Subscriptions and emails then toggle "Send emails when card payments fail" on.