Here are some posts from different sources indicating Garmin Connect did not generate correct sleep summary. It could be a bug in Garmin Connect.
https://forums.garmin.com/forum/into-sports/running/forerunner-35-aa/1412718-garmin-35-issues-and-inaccuracies
https://forums.garmin.com/forum/into-sports/health-fitness/vivomove-hr-aa/1408390-sleep-tracking-messed-up-after-sync
https://forums.garmin.com/forum/into-sports/garmin-connect/troubleshooting/1412092-inconsisten-sleep-data-between-garmin-connect-mobile-and-garmin-connect-on-web
https://forums.garmin.com/forum/on-the-trail/wrist-worn/fenix-5-5s/1409826-sleep-tracking-rem-errors
https://forums.garmin.com/forum/on-the-trail/wrist-worn/fenix-5-5s/1411670-fenix-5-garmin-connect-and-sleep-data
https://forums.garmin.com/forum/into-sports/health-fitness/vivosmart-4-aa/1409947-long-inaccurate-sleep-hours