My galaxy fit has a tendency to freeze and/or reboot during workout tracking. As a result, I miss the majority of my workout data.
Sometimes it just resets. Other times, the screen will freeze with the heartrate sensor flashing. It doesn't respond to any input except holding the side button down for 5 seconds, which resets the device.
The time at which the bug occurs appears to be random (although always when I'm tracking). Sometimes it can track for 3 hours without an issue, and sometimes it will freeze after 15 minutes.
I've got the latest firmware, R370XXU0ATD2. This has happened since I first got the watch (which includes the previous firmware).
I've recorded the behavior in a video:
I would recommend resetting your watch. Give this link a try: https://www.samsung.com/us/support/answer/ANS00084702/