The problem of measuring heart rate while cycling

Completely agree. I think the users reporting issues do have them. 3 years ago, i tried two 6S. the wrist HR was awful. Going up and down the stairs to increase my hr, it would not go over 80 while my chest strap would show 110. I do wear the watch higher on the forearm too where there is more meat. A friend of mine had tried the same thing and his was fine. A year after i bought a 6 Pro to give it another chance and this one has been close to perfect with wrist HR. I think it could be different manufacturers of sensors or maybe flaky ones. Just a speculation though. The thing is that users reporting that may be a very small percentage of the population.

My F7SS worked fine until the last update. After the update, I have the same issue as described here, although I have to admit I never encounter it after approx 5min. When I 'open' (not start) the activity prior to cycling (during getting ready), this indeed partly solves the issue; the steep step from ~70bpm to the actual ~140 occurs 'already' after a minute or so. If I 'open' the activity only seconds before actually starting to cycle, the steep step occurs after 3-5mins.

But, main point; this started after the update. No issues before the update. So I believe there is something to fix.

I disagree. I cycle 3 times a week and my heart rate on the watch will match my chest strap values that i see on the phone. A little lag when i do sprint though but i would say 3 BPM difference. I have a 6 pro. 

I'm not arguing that my chest strap is more accurate, I'm just saying that I didn't have this issue prior to the update, an now I have it.

Seems like something changed due to the update, i.e. the OHR is currently less accurate than it used to be (so comparing with itself, not with a chest strap)

As you see; same issue:

Not that definitive. OHR works very well for some but not for others. That might change for some when OHR is updated. This has always been the case since OHR became a thing. 
What is true is that if you want a better guarantee of regular good HR when active, get a strap. I find more often than not that OHR works for me for any activity but I’ll still use a strap

Its not. F7 is using a different sensor. You cannot expect the same results for your anatomy of your arm.

Buy a HR belt and you're good. Or go back to 6 if that sensor is better for you. 

Different sensor. Different anatomy of your arm compared to the one complaining. 

Sorry for going off topic. Seatanus, what's that multicolor graph above HR? 

I've had a similar problem on 4 different Garmin whist HR devices transmitting to 3 different Garmin Edge devices over the last few years. Sometimes it does appear to be made better or worse with firmware updates but they all did it. Sometimes not every ride and sometimes would be fine for hour after hour but they all did it eventually. I see it very much as both a hardware and software issue. The hardware is not very good for consistently measuring HR when cycling and the software is poor at coping with this - although any would struggle