Schema Registry unavailability crash loop

A user-events Kafka consumer (Avro + Schema Registry, 8 partitions, 8 instances) starts crash-looping on partition 2 at 10:40 while the other 7 partitions stay healthy. Dashboards: partition-2 lag climbs linearly; the owning instance throws SerializationException: Error retrieving Avro schema for id 4127 then exits, the orchestrator restarts it, it re-reads the same offset, and crash-loops. Recent context: a producer team deployed a new event version this morning that registered schema id 4127, but they registered it in a different Schema Registry instance than the one the consumer is configured against. How do you triage and mitigate?