This is a strange issue. While reading all these complaints I went to my Health app on my phone and found the HR data recorded there every 10 minutes like clockwork so I thought "oh that's good that's one issue I don't have". But then I found that it only records that data while I'm at rest, sitting down. To check this I got up and went for a 20-30 minute walk and the last recorded time was prior to my having left the house. Sitting back down now it's gone back to every 10 minutes.
Now if I engage the exercise app it records every minute and actually multiple times a minute.
Even though this appears to be a bug it also appears to be deliberate. Almost as if it's been programmed to only record a "resting" heart rate unless you've engaged one of the activity/exercise functions in which case it records multiple times a minute. But it works consistently for me - sit around doing nothing, every 10 minutes - get up and walk around for whatever time, nothing, - engage an activity/exercise app, every minute/multiple times a minute.
I've checked this quite a few times and it's completely consistent. That really seems like a deliberate setting.