Even expensive wrist watches are pretty poor at Heart Rate measurement. It’s not because the technology isn’t good, it’s just less accurate because of placement, sweat, and many other biological factors. Invest in a good chest strap. It will pay for itself in correct training.
I regularly ride with my Garmin Fenix 7 Pro on and it gives wildly different heart rates than my chest strap. The watch is more accurate when resting and hanging out, but it does a pretty terrible job during activity. The other option is an arm band HR meter that goes around bicep. Those are still better than watches.