In my experience, FitBit Charge HR 2 was useless. Same issue with you where it would 40 bpm off or provide no reading at all.

Apple Watch 3 was pretty bad too. It would give inaccurate often enough that I couldn't use it for training in a heart rate zone. For example, I'd be walking for warm up and it would give a reading of 160 at the beginning, then back down to 110, then 10 mins later back to 160. I can't imagine the Apple Watch being a medical device with lives depending on it.

Garmin 935 has been great. It has the most accurate HR and GPS readings.