If activity data is synced while the phone is locked, Health Sync will add the GPS data to the Apple Health workout data the next time you open the Health Sync app.
With Garmin Connect you can sync the last five years of activity data, and the last two years of other data (steps, sleep, heart rate etc.). With Polar Flow you can't sync historical data.
Health Sync currently supports syncing from Apple Health, Fitbit, Garmin Connect, Huawei Health, Polar Flow and Strava to Apple Health, Fitbit, Intervals.icu, Runalyze and Strava.
Health Sync synchronizes data between health and fitness apps.
If you can't resolve the issue, you can send a Health Sync problem report (the last option in the Help Center menu) or email [email protected]. You will receive support to solve the sync issue.