Originally posted here: https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-55-series/270562/long-list-of-issues-bugs
If you stop and resume a workout, the timelines in Garmin Connect cut the part where the activity was stopped and display it as continuous. This means the activity timeline doesn't match with other graphs (heart rate, for example) nor with actual time:
The middle part (where heart rate is lower) is when the activity was stopped. You can clearly see when the workout actually ended from the heart rate graph.