Vendor schema change silent failure

Your fraud-scoring pipeline calls a third-party risk-signals vendor for each transaction. There is no outage: the vendor returns HTTP 200 on every call, latency is normal, your error rate is flat at baseline. But starting around 15:00, your auto-approve rate quietly climbed from 88% to 99% and chargeback-flagged volume is creeping up in the back office. Dashboards: vendor call success 100%, p99 normal, no timeouts, no 5xx. The vendor's status page is all green. The only context: the vendor emailed last week about a 'minor schema update' to their response. How do you triage and mitigate?