Debug a driver assignment bug

Given a service that selects the best delivery driver ("dasher") for an order, users report incorrect assignments. With a provided codebase and failing scenario, reproduce the bug, write a minimal failing unit/integration test, pinpoint the root cause (e.g., stale location cache, incorrect sort comparator, time-unit mismatch, concurrency/race), and implement a fix. Explain your debugging process, the fix, and its complexity. Add observability (structured logs, metrics) and safeguards (timeouts, retries, circuit breakers). Discuss edge cases: no candidates, ties, GPS jitter, late position updates, partial outages, and performance under high load.