Wrist-based heart rate reliability worse since new updates

This is not completely true. I've used fr245, fenix6 and now fr965. All 3 watches were very good for running HR, except for short, fast intervals, when they all had a lag, which is understandable. But for "normal" runs there were almost no differences between them and the Polar h10.

But as you wrote this has changed now. And the watch sometimes measures 20-30 bpm lower than the Polar. Usually I'd say: ok, but how you know it's not the Polar that has some problem? Well, because when I run at an easy, steady pace, and I feel my breath and HR are not changing much in the middle of a 5k run, and I see the Polar displays the same HR and fr965 displays a 20bpm drop, then there is a problem. Moving the watch, even cleaning the sensor (as much as one can during run) doesn't help.

Something is very wrong with it lately

The heart rate monitor on my 965 has become completely unreliable since the last update, jumping all over the place, I use the HRM pro+ with my running/cycling activity’s, i have been noticing strange readings since the last update. I only changed the battery in the strap last week, today on a run it was stuck on 120bpm when I know it should of been at the high 140s, I disconnected the strap and reconnected, jumped straight up to what it should of been, I’m now thinking the strap was never connected and the 120 readings was coming from my wrist, I’m just back from a walk, only had watch on, again HR was jumping up and down, never had an issue with it before, but have heard of the horror stories, well now looks like I’m in that story now, Garmin get it stabilised there will you, your at this racket long enough now. 

That's what happened to me as well. I thought I'm looking at the HR from the strap, but it turned out the whole activity I was looking at HR from the watch. You can also check it on https://www.fitfileviewer.com/ You should see heart_rate , wrist_heart_rate and external_heart_rate. 

I have also seen very poor heart rate accuracy recently (on my Forerunner 955, but I assume the same HR software as the 965). At first I put this down to the colder weather, but it wasn't so bad in previous years.

Each of my last three activities has had serious issues with the heart rate measurement:

- A run started well, but the HR drifted down from 150 to 110-120 after ten minutes and slowly returned to 150 over another ten minutes.

- A walk, HR dropped suddenly from 100 to 80 when climbing a hill, and recovered suddenly at the end of the hill.

- A run, with threshold interval - HR was maybe 30 bpm too low for the first 16 minutes, with almost to response at all to the start of the interval.

Generally, the accuracy is quite good for me - these three consecutive activities were the worst I have seen this year.

can you upload your fit file to https://www.fitfileviewer.com/ and then look at the Charts and display the heart rate and external_heart_rate / wrist heart rate so see which (external or wrist) was used in heart rate? (maybe upload the screenshot here)

I have the same problem, heart rate 15 or 20 beats lower than my chest strap and my arm heart rate monitor, I thought it was a fault in my replacement unit.I'll wait to see if a new update fixes this; otherwise, I'll have to contact Garmin to have them check my new watch that they sent me.

The capture was yesterday; the area where I run has many hills, it's a very undulating area.