Heart rate drift

I'm seeing my 955/S straying from HR and locking in on cadence.  Prior to 12.23, I don't recall having this issue, but seems to be more prevalent lately.  I had this issue on my my FR645, especially on colder days.  Generally the only way I've found to combat is ratchet down the strap and let the watch soak in GPS and HR before starting my run.  Definitely annoying b/c it will make a slow pace look like I'm laboring then biodata is all off.   You can test if your 955 is grabbing the cadence by slowing way down and speed up your cadence like running in place.  Sometimes, I've even had to pause my run and watch the HR dramatically drop back to a warm-up Z1-Z2, then slowly enter the run again.         

I notice the same if I use the OHR  ,I always rely on the HRM.

I would recommend a Polar Verity Sense - I was having problems at the start of runs with my wrist HR - the Verity Sense is rock solid, I just put it on my arm and forget about it, unlike when I wear a chest strap. Haven’t run in really cold conditions yet, though.

I'm also having the issue that the watch is now extremely prone to cadence lock. Unfortunately I don't have a lot of runs with the old firmware so I can't be sure this is a regression. But the HR sensor is preforming clearly worse than my Vivoactive 4 with an older generation sensor.

I typically run with a HRM because I know cadence lock can happen...but seeing it happen basically every run where I forget my HRM is nuts.

Here the same problem, the frequency dropped or increases without explanation in the races and today lying down I just got a high frequency alert, 140 bpm. I turned the clock off and on and it was 60bpm. I measured on another device and it was 60bpm even. Version 12.27 broke heart rate.

Yeah, for me it seems to deviate after a certain time... 30-45 minutes, maybe an hour. 

But that's the case since the beginning from what I remember so not necessarily the last firmware. In the end I almost always run with the polar OH1+ which is a comfortable but secure strap (on the upper arm) because I mostly train on HR zones...

Was hoping I could at least finally use the forerunner for steady runs, but not quite yet I'm afraid in my case

Try to eliminate effect from cadence lock.

ie wear tighter or a band to cover the gaps.

If you use HRM pro or Dynamics RUN to run forget it. Heartbeat is bugged. Not using it for running either, in my case it broke the heart rate sensor.

That does not always work, or at least not for me.  Sometimes I have to come to a full stop watch my heart rate drop like a stone then start again where Garmin grabs the HR and lets go a of cadence.