Since SW 11.11 I've recorded many activities where the heart rate is completely off.
It randomly drops off at some point during the activity and it takes minutes for it to recover.
In all cases, the watch was strapped tightly to my wrist and I the heart rate was locked.
Activities include cycling, running, indoor rowing. No extreme movements, etc.
Providing a screenshots below where you can see my heart rate is shown to be around 65 for first 5 minutes of cycling at 20km/h. And then it suddenly jumps to 130.
Similar situation with the second screenshot.
This is really embarrassing Garmin! I'd expect at least HR to work accurately from a €350 watch that is not suppose to be a toy, but a tool...