Indeed, the Fenix 6 (Pro at v10.10) is a total mess in regards to the heart rate measurement - just caught it bumping my HR with 30% from my real rate while totally motionless.
So, today while measuring my blood pressure with an Omron monitor (first seated then lying in the bed), I saw that there was a difference of 20 beats:
- Fenix showing 80-85 bpm
- Omron showing 60-61 bpm
At the time of measurement I was wearing the Fenix loosely on the right wrist. Tried to move the Fenix around the wrist and tighten the strap but to no avail - 80-85 beats.
I got my Fitbit Charge 2 and placed it on my left wrist; no surprise, Fitbit's measurements were around 60, too. I then changed the wrists with the Fenix to the left and the Charge to the right (making the relevant Wrist settings changes for both devices):
- Fenix at ~80 bpm
- Charge 2 still showing ~60 bpm
After working on my desk for a while, I took another blood pressure measurement:
- Fenix now in the range of 70-80
- Omron/Charge duo showed identical values of 60-ish
A couple of minutes later the Fenix got down to around or less than 70. As of time of last edit of the message the Charge is showing 66 and the Fenix is at 69. I also noticed that just moving the Fenix around my wrist makes the HR jump from 65 to 7x. At some point I also changed the Charge 2 for a Nokia/Withings Steel HR for a while and it was showing good 6x bpm.
I want to note that at the time of my first blood pressure measurement there is no way I was at 80 bmp because I was not half an hour into the day and was feeling so relaxed and sleepy that I have no issues vouching for the Omron/Fitbit being closer to the reality 
I also want to underline that the wrong measurements were being shown on both left and right wrists in the span of more than half an hour while I was experimenting with different wrist positions and with me being first in bed and then sitting on my desk.
The real issue I have in this case is that the Fenix costs as much as the Omron + Charge 2 + Steel HR combined, but the heart rate measurement at utterly trivial conditions (me being still or seated) is so off that I cannot help myself being angry and disappointed.
Please, take this thread not as a question but as a warning to potential buyers and as a bug report. Obviously, no one in the forum can help in this situation.
P.S. It seems that the issue is somewhat related to the "lateral" position of the watch (its position left or right from the centre of the wrist) in regards to the end/most protruded part of the Ulna bone (the outer hand/wrist bone - picture). From my experiments, given the watch is placed in the pit between the two hand bones (where normally a watch is worn and would naturally end up because of the hand shape), if the watch moves to the outside of the wrist towards the Ulna bone then the HR measurement immediately jumps with 5-10 beats. This happens even when the lug of the watch is still not protruding from the outside of my arm, meaning it is not rotated around the wrist.
I managed to get that 80 bpm again (Charge 2: 66) by placing the watch roughly at or below the most protruded part of the Ulna (where a loosely worn heavy watch would end up on its own). I tried doing the same with the Charge, but even placing it directly on the most protruded part of the Ulna (no way it can stay that way there - on top of the dome of the bone almost falling to the side part of the wrist) it was still showing something in the 6x range.
It seems that the only way to make the Fenix have a proper HR measurement is to have it so tight, so as to not move at all. But given the size and the weight of the Fenix this is definitely painful and not something I'd count as "by design".
Maybe it has to do with the two (?) sensors, like one getting obstructed and throwing the measurements off? Because unlike the Fitbit, where there are 2 LEDs and 1 sensor, in the Fenix I see 2 LEDs and 2 sensors. Whatever the reason, the "state-of-the-art" part of marketing in the HR Accuracy notice should definitely go.
