Optimistic lock livelock

An inventory-allocation service that uses optimistic concurrency (version column + compare-and-set, retry on conflict) gets stuck during a flash sale. Dashboards: CPU is high (60-80%), the service is busy and threads are running, nothing is blocked or deadlocked, yet successful allocations per second drop almost to zero. The retry-counter metric explodes: the average transaction is retrying 40+ times before giving up. The DB shows tons of version mismatch conflicts on a few popular SKUs but no lock waits. Restarting doesn't help. Triage and explain why it isn't a deadlock.