Order enrichment consumer lags

At 14:20 PagerDuty fires: the order-enrichment Kafka consumer group (12 partitions, 12 instances) is lagging. Dashboards: end-to-end lag climbed from ~3k to 900k over 25 minutes on every partition roughly evenly. But unlike a normal lag event, the broker-side produce rate on orders is flat at its usual ~4k msg/s, no traffic spike. The consumers are at ~8% CPU and almost idle. Each consumer does one synchronous HTTP call per record to an internal customer-profile service; that service's p99 latency went from 20ms to 1.8s at 14:15 (its own dashboard shows a slow dependency on a Redis cluster doing a failover). max.poll.interval.ms is the default 5 min and no rebalances are firing yet. How do you triage and mitigate?