Coming into two months of ownership of the 955 now - everything's been brilliant until recently where I've started encountering issues (might be update related).
Starting with heart rate readings, it's something I always keep an eye on and quite often use as a reference as running. When I first got the watch, I compared the readings to my Wahoo Tickr and found the watch was coming in extremely close to what the strap was reading throughout exercises. From then, I've been pretty comfortable trusting the data picked up by the 955 for the odd zone 2 heart rate run. The past few weeks; however, I'd say 3/4 of my activities are coming in with heart rate errors somewhere along the way. What seems to happen is the watch will plateau around 140bpm for a couple of minutes before correcting itself. In this period, my actual heart rate could be 60bpm or it could be 170bpm, but the watch always seem to gravitate to around 140bpm. See a few examples below:
30/03/23: 5k race done at maximum effort. Typically for these my heart rate will site high 170bpm. Referring to the below image, the watch had my at 140bpm for the first 12 or so minutes before jumping and correcting itself to around 180bpm for the rest of the race.
30/03/23: On the warm up for the above race, the watch threw my heart rate up to 140bpm at the start (would be lucky to be over 100bpm at this point) and plateaued around that number. I did a couple of strides (as well as other drills) during this warm up which usually always picks up my heart rate. See below graphs compared to an equivalent run done the day before which contained strides. 
Equivalent workout from the day before:
28/03/23: Attempted an easy run (based on heart rate). As soon as I started, my watch jacked my heart rate up from 70bpm up to 135bpm and did random plateaus from there, none of which correlated to the effort I was putting into the run. After 8 minutes, the watch corrected itself and suddenly dropped the heart rate by 30bpm.
21/03/23: Another easy controlled run intended to be done by heart rate. Everything was good and working until around 24minutes in when the heart rate readings spiked and plateaued (immediately dropped my efforts but the watch was doing its own thing). After 4minutes of errors, the watch dropped my heart rate from high 140bpm down to 70bpm where it, again, plateaued before fixing itself a few minutes later.
Next up is structured repeat training intervals created through the watch. An example from a few weeks back, I created a 10x400m on/off track run workout with warm up and cool down. For some reason, the watch would skip the 400m off, but gave me the full distance on my watch saying I did it in the time it took the watch to skip the off interval (milliseconds). The result of this was my workout ended up 4km longer than what I actually ran, and at world record paces. I had to manually correct everything but residue of the errors are still shown in the splits.
It's also worth noting that on the above, I had a primary race widget set up on my watch (target was 16:59 for a 5k). After this workout, it dropped my predicted finish time from 17:XX to 9minutes. Despite deleting the activity from Garmin and my watch, my watch would not ignore this false reading which messed up the training recommendations and predictions until the race was complete.
