This has been going on in the last few versions and even in the most current version. Whenever I do a treadmill activity the HR shown on the watch goes around 100, where as my actual HR could be around 150. If I run very fast on treadmill the HR on the watch would go up to 130 - 140 when my actual HR could be 170. It is usually not until 15 - 20 mins in the HR on the watch starts going up closer to my actual HR. This is extremely weird. I thought it was my wrist but i have tested on both wrists they yield the same result. However, if I use the walk activity on treadmill the HR on the watch the HR would go up right from the start. I don't know if there is a fault with the activity itself. Otherwise I will have to use the walk activity for treadmill. (Fenix 5S on latest firmware)
This is a known problem for firmware 6.0 and on as reported by Garmin tech support. They recommended that I copy the running app and disable GPS, but that didn't really help either. An upgrade to 6.75 seemed to help, but not alleviate the problem. If it's any consolation at about 10 minutes into the run I start to get (what I think are) correct readings.
