HKT cohorts and API tool return ghosts
Day-one engagement can move when clocks and user IDs move—not when the API tool does.
Teams serving developers in Hong Kong often mix UTC warehouse timestamps with HKT product calendars. A cohort day that spans the wrong midnight shifts every engagement cell without anyone shipping a feature.
SDK upgrades that change anonymous-to-known stitching create similar ghosts: returning console users appear as new cohorts, and week-four rates look healthier or worse depending on merge rules.
Before celebrating an API-tool engagement lift, list SDK versions, identity rules, and timezone settings beside the chart. If those moved, treat the chart as suspect until you rebuild the cohort from raw events.