In answer to your original question, chest strap HR monitors are very accurate*, optical wrist ones less so.
The purple line is the HR and,as you can see, it appear to spend quite a lot of time over 150bpm over the earlier part of the ride.
*although only accurate with good electrical connection to your chest. Abnormally high readings early on in a ride/run are a typical sign of a lack of good connection. Once you’ve built up some sweat (and electrical connection) the problem goes away. Plenty of spit on the chest strap and chest solve this, or buy some conductive gel off ebay. High voltage pylons can also screw up the readings, but it’s mostly just lack of moisture between the strap and your chest.
If you see a high HR, ask yourself “Am I out of breath?”, if not, it’s just a garbage reading.