ran a half marathon pace 99% hr zone 4. After update ran intervals at faster paces and didnt even reach hr zone 4 and I was definately i zone 5...
Another forum member said that the watches in the new software versions use an average heart rate over several seconds. This of course makes the evaluation of interval training worthless. I have no guarantee for this statement, but I have the same problem with my 255. Sprint inserts have recently been completely ignored by the heart rate monitor.
Just buy a chest strap. Sincerely, your Garmin sales agent /s
I'm sorry to hear about that. Please see 18.20 HR bug still here for the main thread regarding this topic.