HRV has been poor since the update to 20.18

Nope, that was what i meant with " i cant explain this physically"

No illnes at all

Well, my HRV has been going up the last two weeks - which is explainable to me, as I was on vacation and hiking but nothing else (=low load) and sleeping long and good.

So personally I cannot confirm, it might be a bug, at least not on my FR955 with FW 19.18

Are you sure it started right away after installing the new firmware? You installed it after the 5th of June, then?
Did you reboot the watch or even made a "soft reset" (=hold upper left "light" button for around 15 sec until the watch shuts down, wait a little, start it again - this is supposed to also clear caches and stuff...)?

Of course, I have restarted the watch....also several times because of the HR problems .

I'm 95% sure that the problems started with the update

I agree that something is not right with the recent updates. In fact going all the way back to an update in March ( which I think included the new "nap" function) apparently I no long sleep in the REM stage. Same with HRV numbers, they do not represent my actual feeling of recovery and wellness. Personally, I just hide everything that uses sleep and HRV and just use the watch for tracking my runs. Doing this has actually decreased my the stress I "feel"  . I just use the Morpheus App and a Scosche rhythm 24 arm strap to track my recovery every morning now, way more accurate! Pretty disappointing that I have to resort to multiple devices considering how much we pay for these watches.    

That's exactly it, I also think that it doesn't reflect my body feeling.

I had a brief upswing last week, but now it's gone down again.

 A few months ago I told everyone that the Garmin data always made sense to me. But for some time now the data has been incomprehensible to me

I think the optical HRM is garbage and Garmin keeps trying to adjust the algorithm to compensate for the inaccuracy. The sad thing is you can buy a far more accurate HRM for much cheaper. I have own many Garmin watches, but, prior to the FR 955, I tended to stay with the cheaper models (e.g. FR 235). Personally, I will not buy a higher end Garmin product again.  

I've been following the Daily Suggested Workouts for a while now.  Since I started doing that two things have happened: My training status has moved to "Productive" and has stayed that way, and also my daily HRV has completely stabilised in the middle of the range, increasing very slowly.  Also my overnight HRV averages do not swing so wildly as they did before when I was training more erratically.  This all makes sense to me and suggests at least in my case this part of the watch is working as expected.

I did that for a while, but I don't get enough rest and I injure myself too often. So now I train every two days at most and then I usually, but not always, stick to it

Now another nice example, my HRV has now remained relatively constant. 

Now I've had a bad cold for 3 days with a fever of up to 40°C and the HRV remains where it is and I'm supposed to do Vo2Max training