Inaccurate readings wearing arm HRM

I purchased the COROS arm heart monitor as an alternative to my chest strap to pair with my Garmin watch. When I’m not running it seems it’s reading correctly (I manually measured my bpm) but when I do run it is reading anywhere from 15-30 bpm higher. I tried tightening, loosening, adjusting placement and nothing seemed to work. Disconnected and reconnected multiple times and that didn’t help either. Anyone experience this? I’m over wearing a chest strap and was hoping this would be a reliable alternative.