I bought the watch, had the same issues for 5 days and I returned it. But my friend who also bought one at the same time with me didn't have these issues(hr is constantly recording) so he kept the watch.
I asked him to give it to me for the day just to see if his watch would work on me also, because I really liked it and would buy it again if it works and just my device was defective.
In the beginning it worked, I was even moving it around to see if some spot on the hand would cause trouble, but no, hr was constantly recorded. And then I didn't tackle with the watch, but just wore it along the day and when I looked at it (just at some random after 2-3 hours), it had already stopped recording 20 minutes ago. I tried to move it on the wrist to see if it will start tracking again, but no, completely stuck. And as always, I had to go to HR settings - change to Frequent, do one manual measure, then go to settings and switch back the HR setting to Always On. Then it started working again.
Pretty weird..... We tried this for 2 days and on my hand this happens just on random, while on his hand the HR readings never stop. And we wear it in the same fashion in terms of tightness, spot, etc...