My Fenix 6 says the HRM Pro+ battery is fine. I wear a cotton shirt, so no synthetic sparks. The Fenix reads the HRM Pro+ fine for a few minutes, but after about 15 minutes, the Fenix receives no data unless I hold it up against my chest 2 or 3 inches away from The HRM Pro+.
I get more consistent data using just the heart rate monitor in the Fenix.