Optical HR not reliable

I also suffered a lot because of inconsistency of Optical HR.
It is not reliable, period. You can do nothing about this.
I finished up buying chest heart rate monitor, it has electrodes (like in electrocardiography)
and heart rate is measured perfectly.
So my advice to you: if you need correct HR - do not rely on optical HR.

Thanks. I was thinking about it but now, HR monitor are embedded in the chest strap so when the strap it torn, you have to buy another one. On my old model, you could change just the strap. I don't understand why when selling a 1000 Euros watch they cannot provide an HR chest with it... I refuse to give again monney for an HR monitor that will last just one year.

Do you think it's a software issue or hardware issue ?

Welcome to the club falconeye75 !
As jemcik is saying: if you want reliable HR monitoring during activity, chest strap is the only bullet proof way to go.

Optical HR is good for 24/7 monitoring, but not much more.

This is not unique for the F5X+ or Garmin, the limitation is first and foremost in the technology (hardware) but some strange results may also be caused by software (often when developers try to implement «smart» software compensations/guesswork to make up for poor measurements)

Hi,
...HR monitor are embedded in the chest strap...

I use wahoo tickr x, absolutely happy with it,
works nicely with fenix 5x plus,
and yes, it has separate body and the strap.

I had a lot of issues with the Wahoo Tickr X, the heart rate variability doesn't work correctly over ANT+ and sometimes it would flake out and tell me my heart rate was crazy low or less. I recently got a Polar H10 and it works extremely well, and in the latest firmware it can connect to two bluetooth devices (so my Phone and the watch at the same time, helpful for Zwift and indoor biking apps)

My 2 cents : the Optical HR is indeed not very reliable, it was too high for my first run with a 5X plus. I should wear the watch a bit tighter. But I still don't think I will use it for workouts. Instead, I bought a Scorsche optical device, you wear it on your forearm or arm, it's very lightweight and stable, you don't notice it. Much less than the chest strap or a tight watch strap on your wrist. The reviews say that it's very reliable, maybe not as much as a chest strap, but it's quite in the same league. It doesn't suffer from the slow start of the chest straps for instance. I will pair mine with the 5X for workouts, and keep the optical sensor of the watch for the 24/7 monitoring. It's an extra cost, but since I tried it, I don't see myself wearing a chest strap again (and I'm coming from Suunto, their belt is very comfortable, but I still had to readjust it...). The latest version from Scosche can do HRV at rest, so it can be used to monitor your recovery. Quite a nice device !

I have the same problem but with 5x.
It started with the new software and before that I did not have a problem. he was relatively precise. so I do not think people are saying correctly that this is a general problem with optic HR

Did you know with each version you noticed this problem occurs ? Maybe it can help beta team. For me the OHR is not usable in its actual version as soon as you are modifying your run. When maintaining your speed for 30-60s it is ok but it is like it take into account HR variation after a very long delay.

I found the same thing with MTB bike - slow to pick up on changes - like if I stop briefly and then move on, doesn't catch the drop in HR and the pickup again quite right. Sometimes takes a couple of miles to kick in etc. I had the same thing all the HR watches though so I bought a strap a long time ago.