Why was the same workout re-sent with a different timezone?

Apple Health timestamps use the device's current timezone at sync time, and historical timezones can't be retrieved on iOS.

Apple Health timestamps use the device's current timezone at sync time, and historical timezones can't be retrieved on iOS. So a workout backfilled after travel can carry a different offset.

Duplicate workouts aren't deduplicated by default. Dedupe in your own pipeline on the workout's unique ID so a re-sent copy with a different offset is treated as the same workout rather than a new one.