It's definitely a bug:
This was just reported on the apple forums:
"I just received a phone call from someone at Apple about this issue after I posted about it on Twitter. They took all of my information, spent about 30 minutes fact gathering, and has had me send them my heart rate information from the Health app to help them diagnose the issue. I was promised a follow up in the next couple of days with an answer to resolve the issue.
In going through the data, we found that it is not reading while actively moving. While sleeping or being very sedentary, it is reading every 10 minutes as it should. However, while even walking while on the phone, it was not reading for well over 30 minutes or more."
https://discussions.apple.com/thread/7050353?start=30&tstart=0
I just did a test for walking around the house and the HR was recorded every 10 minutes on the button. So, obviously the HR monitor is working differently for different people... Probably based on whether it has been re-paired or re-booted. I have done both.