Watch Ultra 2 - Heart Rate monitor way off

Ultra2 says HR is 114, Treadmill (Mostly accurate, did manual check) says 144.

The monitor is way off when exercising, meaning when I'm exercising on my own rather than in a gym, I can't rely on it. It's also harder to fill out my "Hearts" or whatever for Raised HR activity. I was at 130-150 BPM for 55-56 minutes for cardio, then did several sets of various exercises (legs, core). It only recorded 40ish minutes of raised HR activity during my 2hr workout.