Hi,
already raised in other Posts (see list at the end)
and tried all of this:
The Heart Rate Sensor on My Garmin Watch Is Not Accurate
Problem is not accuracy, the heart rate is sometimes completely wrong.
I went back into all activities (Garmin connect shows the watch + firmware used in each activity) and came to this conclusion:
firmware 10.x 11.x : ok (not as accurate as chest wrap but ok)
firmware 12.x + later : not ok (unexplainable values going too low / too high)
it's not the temperature (cold) ; it's not the cadence lock ; it's not the position/tightening : ruled that out and did many tests.
If you can confirm your findings (look in history + which firmware) then we can raise this as a firmware bug to be solved