I've had my 5 Plus for a little over a week. I use it a lot as a HR strap in conjunction with my Edge 1030. The first week, the HR rates seemed consistent with what my strap had been reporting based on effort. The last 2 days, it's been all over the place, telling me my HR is 105 on an all out sprint. Even on a hill climb, I could not get it to read above 157 when I knew i was much higher.
When I got home, I threw on my HR strap and fed it into the Edge and left the Fenix broadcasting. I did about 5 mins of jumping jacks; enough to get my HR up. Strap read linear and based on effort; Fenix stayed about 30bpm below and not reactive.
I tried rebooting it; changing wrists, flipping it to my wrist. Nothing changed.
Do I have a bad unit or did the most recent update bork it?