I had the same Rebooting problem. It Happened mid or straight after exercise or post a 2 min shower. My Wrist sweats a lot and I think the sweat would somehow get into the watch.
Even though it says Water resistant the Wefix repair man said (we recorded the telephone call) it’s just something Samsung says, but the watches aren’t water resistant AT ALL.
Took it to Wefix for repairs, they replaced the motherboard 3 times. Now I’m in comms with Samsung and the third party who sold me the watch and I’ve demanded a refund or a new watch (not the same one obviously) as it’s still within warranty.