With the previous firmware I had a lots of issues with the HR belt (Suunto smartbelt), tried changing the belt, battery etc. Nothing worked. Instead I bought a Wahoo Tickr (Ant+ and BLE) to use. Haven't had a single problem with the Tickr. Did a 12 hour ultra race last weekend and it worked perfect. Not sure where the problem is. Is it the Spartan or the belt? Or a combination? My Smartbelt did however work with my old Ambit 3 without a problem.
For those of you that have a problem with the Suunto HR belt. Do you have the same problems if you use it with another device (not a Spartan)? Eg, a phone app such as Movescount, Wahoo, Strava? That could rule out the HR belt as being faulty.