I have exactly the same HR issue... See my topic I created for this (FORERUNNER 245 - HRB update too slow during activity)
I've had the HR issue as well. The interesting thing is I had the same exact issue with a VA3M before I returned it. I would start an activity and HR would float around 100 BUT once I stopped the activity and then restarted it, the HR would jump up to where it should be. Seems to be software related but has been going on for way too long. It's hard to see a pattern though because sometimes HR works from the beginning and sometimes not...