According to various reports on online forums and Apple’s own support forums (via The Verge), users are complaining that the device basically stops measuring the wearer’s heart rate for long periods of time. Usually when one is in a more idle state, the Apple Watch will take the wearer’s heart rate every 10 minutes, but ever since the update, the timing is all over the place.
Users have submitted screenshots in which it shows the log of the heart rate where in some cases, the gap between measurements are about an hour. Sometimes it changes to 30 minutes, sometimes it’s 40 minutes, and sometimes it appears to be working normally by taking measurements every 10 minutes.
That being said the heart rate monitor appears to be working fine when used during workouts or via Glance, it’s just that its idle recording state seems a bit messed up. For a device found to be almost as accurate as a proper heart rate monitor, this certainly seems a bit disappointing. Hopefully Apple will have a fix for this problem soon.