Buildkite change history: Delayed notifications

Buildkite reported a major incident affecting a subset of customers, with delayed email, GitHub commit status, Slack and webhook notifications; the postmortem also noted build dispatch and metrics latency. It started at 2026-05-20 16:40 UTC and resolved at 2026-05-20 17:39 UTC, lasting 59 min. Buildkite applied mitigations and monitored recovery.