Try to place the watch well over the wrist. Suunto’s HR readings seem to be very affected by that. I come from a Spartan WHRBaro and we’ve never been friends in relation to optical HR measurement. I remember once that I forgot the chest HR strap and has to rely on optical when I was just doing easy warm up at -supposedly- 192bpm…
Now I have a Polar Vantage V and it reads HR properly with the watch where a watch should be places, not that over the wrist. 24hr HR reading is spot on.
There must be some reason why I have such different experience with these two well-respected brands. The Vantage is smaller and lighter than the Suunto. So I guess that could be a key factor? Who knows…
Give the different placement a shoot and see if that helps!