WatchOS 1.0.1 issue, which effected the heart rate monitoring after every 10 minutes to work only at random intervals, is not a bug, says Apple. Apple’s updated support page, spotted by 9to5mac, says that it is not an issue and intentional behaviour. Heart rate monitoring only works when the arm or hand are not moving.
Why has Apple chosen to make this change, without notifying users or mentioning it in the WatchOS 1.0.1 change log is actually surprising. Heart rate monitoring on Apple Watch even works when you are in constant movement during a work out and using the app to measure it. So if Apple is trying to convey that this change is to get more accurate readings, it seems incorrect.
However, it seems likely that this is the reason why Apple Watch battery seems to be lasting way more than before because the checks are less frequent. I have even noticed gaps of multiple hours between different readings.
It would be better if Apple makes this ‘feature’ optional rather than enforcing one way or the other. There are users who would like consistent readings, and if that comes at the cost of less battery life, let them make the choice.
You can force manual checks by using the heart rate monitoring glance on your Apple Watch but it is not as convenient as automatic readings.