Man, am I glad I found this thread...
This issue started off for me on my older Watch Series 3. Everything was operating fine until the update to WatchOS 7, then as most of you have experienced my watch would unexpectedly lock itself whilst on my wrist. It would happen sometimes after 10 min, sometimes seconds after I unlock it.
After about 3 weeks, I decided to use this issue as an excuse to upgrade my watch to a Watch SE; it happened again. Now, at this point I figured it was something OS related, however giving Apple the benefit of doubt I decided to exchange it for another unit within the return period. I received the 2nd Watch SE and still the same thing.
Now I'm on a fresh Apple Watch Series 6 and guess what? Still locks on my wrist. Super annoying, but I did read through most of these comments and I do have a full length sleeve on my left arm (where I typically wear my watch). Just for kicks 'n giggles I decided to switch it to my right wrist and I haven't had it lock on my unexpectedly whilst wearing it ever since.
Not really sure why it can't read through my tattoos, especially when it was operating just fine prior to the update, but I guess this is the life I'm living for the time being....
I hope Apple steps up and fixes this. Seems kinda silly considering how huge of a company they are w/ all the resources they have lol.