So I've been watching the data gathered from my 5X Plus over the last couple of months, and the graphs on Connect show that there is a higher occurrence of HRM dropout on v10.
Mine are with the cycling and mostly indoor cycling. After 15 minutes, and usually in the early 20 minute range, the HRM will drop to a high resting rate, then slowly start to climb back to normal. Sometimes it happens more than once. It NEVER happens in the first 15 minutes.
I've read all the forums about watch tightness, yada yada. I've been wearing the watch since it came out, tracked all my activities that I use it for, and this has been the only time I'm started to have a consistent HRM issue like this. It's show to be more consistent in v10