On my 5X Plus, the optical HR accuracy used to be acceptable and was almost always in the right HR zone. Now, after all out efforts when my HR should be near max (~180) the watch will read ~100bpm, way off! As a check, I've been wearing my HR chest strap again and HR values are where they should be.
This wasn't always the case, optical HR used to be accurate, but seems to be getting worse. A couple other people I run & ride with have noticed the same problem with different Garmin devices. Did the software change and get worse?