Meta's notification names a submission rather than carrying it, so the answers are
fetched from the Graph API after the response has gone out. Which workspace a
delivery belongs to is decided by page_id.

Every delivery must carry x-hub-signature-256, an HMAC-SHA256 of the raw body
keyed with the app secret. An unsigned delivery is refused rather than stored: the
URL is typed into Meta's dashboard and is no secret.

Meta retries anything it does not get a prompt 2xx for, which is how a brokerage
ends up with each lead five times over — so deliveries are de-duplicated on
leadgen_id.