I recently bought an instinct 3 50 mm solar watch. I've previously used a Garmin Instinct 2 and an Instinct 1st gen. I've noticed big variations in heart rate (HR) and heart rate variability (HRV) readings between these watches. My current Instinct 3 Solar 50 mm consistently shows stress levels approximately 20 - 30 points higher than my previous Instinct 2 Solar, despite both using the same gen 4 sensor. You can look at my HR stress data below when I switched from my Instinct 2 to my Instinct 3, and you'll see a big change in the recorded stress levels for the rest of that day.
You can also see little white things in the glass, look like scratches even though it's a brand new watch. Wonder if that's affecting it...