I have Vivoactive 4 and since 2 or 3 updates back the device will turn off if an activity starts with the battery lower than 97/98 percent. It will run just fine for the duration of the activity, even a marathon but once that activity is complete you will need to charge it back to 100 percent to attempt a sync.
The device also did not capture correct running speed after the first update, if i ran faster than my normal average it would track me as being slower even if i was just short of a sprint, this is minor compared to the switching off though
It is extremely annoying, I have done a factory reset already.