I just experienced this situation and found out this from user's manual.
"If you do not save the activity manually, the device automatically saves it after 25 minutes."
Bottom line. Worst of all, there is no option to disable this auto-save function.
Bottom line. Worst of all, there is no option to disable this auto-save function.
Not sure about Vivoactive HR, but on my watch I can pause an Activity, select the option "Resume Later", and resuming it hours (or even days) later, with no auto-save happening in the meantime.