Consumer group rebalance loop

Starting 03:50, the notifications Kafka consumer group (24 partitions, 12 instances) enters a state where lag oscillates wildly, climbing to 2M, dropping to 200k, climbing again, and never stabilizes. Dashboards: the group's rebalance-rate-per-hour JMX metric is pinned high, instances log Member X sending LeaveGroup ... consumer poll timeout has expired and Attempt to heartbeat failed ... group is rebalancing in a loop. CPU is moderate, no OOMs. Recent context: yesterday a new feature added a per-message call to a templating service whose p99 is ~600ms, and message volume is normal. max.poll.records=500, max.poll.interval.ms=300000 (default 5 min), session.timeout.ms=10000. How do you triage and mitigate?