KST cohorts and the wrong midnight
Day boundaries move retention cells when warehouses stay on UTC and product thinks in Korea time.
Application teams in Korea often mix UTC warehouse stamps with KST product calendars. A cohort ‘day’ that straddles the wrong midnight shifts every retention cell without a single feature shipping.
SDK upgrades that change anonymous-to-known stitching create similar ghosts: returning users look new, and day-seven rates swing for pipeline reasons.
Before celebrating a lift, list SDK versions, identity rules, and timezone settings beside the chart. Session Continuity Reviews open with that checklist on purpose.