Not sure what’s causing your issue with your Garmin’s wrist heart rate monitoring. From my experience the Garmin FR 945 has the best wrist HR monitor. However I use the HRM pro 95% of the time. I love how the "features” on the 945 actually work compared to Polar and Suunto. Hopefully you find a solution to your problem.
My solution was to wear a strap ~2 in wide to keep the watch further up the arm. Helps eliminate faulty result due to wrist flexion. Over a year comparing chest strap with wrist HR showed very good (of course not perfect) correlation when worn this way.
I fiddled with a few different things, cut up sock, old man wrist sweat bands (being old myself, I had a set), which all worked well enough. Eventually purchased elastic material and sewed velcro to it. The band starts where the wrist creases, and the watch is placed above where the band ends. It isn't the "2 inches above the wrist bone" that the manual suggests, but it seems to make all the difference.
