My series 3 Apple Watch never spontaneously rebooted until watchOS 7.0 and still occurs after the 7.0.1 update. I tried the unpair - pair my watch ‘fix’ and while better, the problem still occurs.
Today I observed and important clue about all of this.
After walking my dog this morning, I went to shower and placed my watch on the sink face down. The heart rate green LEDs are on. That’s odd since they normally go off when the watch detects it is no longer on my wrist. Waited a minute and the LEDs are still on. Put the watch back and had to unlock it (watch knew it was no longer on my wrist) and had it take my heat rate. When that was done, I took off the watch and the LEDs stayed on for another -5 minutes before finally turning off. I have never observed behavior like this before. If this sort of behavior is occurring while on my wrist, no one would even know it. The brightness of the LEDs tells me they would run through the battery charge rather quickly as it has a small battery. I see a lot people are reporting reduced battery life in addition to the reboot issues.
I also wonder if this could be related to reboot issue too. The Apple Watch is very small package that most likely has some thermal limits. Normal operation has it placed against the human body the can be upwards of 90 degrees. The stuck on LEDs would drive up the temperature even more. What does the Apple Watch do when it’s internal temperature gets too high? I would not necessarily feel this on my wrist due the fiberglass back for the heart rate sensor and I might not be in contact with the metal part of the case at this time (now I might expect this isolation to work both ways, but internal temperature is what matters here). In more than one instance of rebooting, it took a minute or two before I saw the Apple logo appear (I kept hitting the buttons trying to bring it back to life during this time). Could the delay be the watch is simply cooling down enough and then it comes back to life?
One other observation, when the reboot issue started, my local weather was hitting the 80’s and now it is just hitting the 60’s. I am seeing fewer reboots now.