Consumer lag after deploy

PagerDuty fires at 02:14: the orders-enrichment Kafka consumer group is alerting on consumer lag. Dashboards: end-to-end lag on the orders topic (12 partitions) has climbed from a steady ~2k to 1.8M records over 40 minutes and is still rising linearly. Broker-side produce rate is flat at its usual ~9k msg/s. Consumer group has 4 instances; CPU on them is ~30%, and the consumer's records-consumed-rate JMX metric dropped to near zero ~35 minutes ago. The group's rebalance-rate shows a spike right before the drop. A deploy of the enrichment service went out 45 minutes ago. How do you triage and mitigate?