Kafka consumer underutilization on scale

A Kafka topic webhook-delivery (8 partitions) feeds a consumer group running on Kubernetes with an HPA targeting CPU. At 19:00 a marketing send triples inbound volume; lag climbs to 1.2M and keeps rising. Dashboards: the HPA scaled the deployment from 8 to 30 pods, CPU per pod is now LOW (~25%), yet lag is NOT draining. Only 8 pods show assigned partitions / non-zero throughput; the other 22 are idle. No errors, no rebalance thrashing. How do you triage and mitigate?