I know, it‘s an old story with the heart rate sensor on the hand, and I know also the logical problematic and the technical limits of this technology and I do not expect the sensor of the watch to work perfectly accurate.
A short time ago, I moved from the Fenix 5 to the Fenix 5x Plus and I was upset with the accuracy of the HRM, I bought also a chest strap, especially as the 5x+ did work worse than the Fenix 5 during indoor rowing and strength training (yes, I know that most watches have problems with those activities and it sounds logical to me why, so no complain at all), so with that strap, no problems, works like a charm.
Now, I‘m on a business trip and I do not have the strap with me and I did two treadmill trainings the last two days and the watch does actually stay for the first 20 minutes or so at a pulse of around 100, then suddenly jumps to 140, where it stays till the end after 1 hour complete.
The Fenix 5 was also inaccurate, but did not have such a „jump“.
I also had a problem during some hiking session (where I do not use the chest strap), that the pulse got stuck between 60 and 70 and I needed to reboot the watch and after that the reading stayed acceptable the next hours.
I think I have to agree a lot of people, saying that the Sensor of the watch is fine for 24/7 all day tracking, but is useless for trainings.
I can live with that, but the thing during hiking concerns me a bit and I absolutely don‘t want to watch a chest strap during hiking and the pulse is there also not that important to me, so I‘m afraid I will have to live with that.....
I investigated a lot on the internet about this topic recently and it seems that this is a discussion all over brands and types of watches with built in HRM.
As I tried already different positions on the arm and also different watchstrap settings, I thought I might give it a try and ask around if somebody found a reliable solution, although this whole topic seems also to be very much related to the wearer (skin color etc.).....