We also have problems with the optical heart rate measurement during sporting activities with the Garmin Forerunner 735xt and Fenix 6. With our watches, the problems always occurred after a firmware update (extremely high heart rate values because the watch confuses the cadence with the pulse or measurement stops). On the English and German Garmin customer forum you can find many frustrated posts on the subject of heart rate measurement.
Garmin advertises the HR measurement on their website in a conspicuously restrained manner and on the support page you can find detailed explanations as to why the HR measurement may not work for people with "certain physical characteristics", e.g. when jogging. Interestingly, my HR measurement was better with the Forerunner than with the Fenix 6.
I suspect that Garmin also tries to "improve" the HR measurement with firmware updates, and at best makes things worse for other users. They obviously don't get it right for all users. That's why the RF measurement suddenly only works badly after a firmware update.
The customer support reacts very quickly and nicely, but apart from perseverance slogans and unhelpful advice, nothing is to be expected. They even exchanged my Fenix 6, which is of little use in the case of a firmware problem (save the postage: I had hoped for a replacement with possibly improved hardware; what I received was a watch with a much older production date according to the serial number / delivery firmware version).
Conclusion: either invest another 60-100 Fr in a heart rate belt, or wait & hope for the next firmware update ... or throw the part into a corner in frustration.
I resigned and bought a HRM Dual. Now I train with accurate HR measurement just with a heart rate belt again (which I wanted to avoid with the purchase of a Garmin sports watch...).

I know the problem too. Suddenly the HR sensor only has a straight line at about 90 heartbeats. Restarting the watch always helps. But the problem comes back at some point. I suspect the software for the HF sensor crashes. So I don't know what the final solution is.

After using the Forerunner for a good 2 years, I also had problems with the HR. It was always 170+ for me during an activity! Since the warranty period has just expired, I have switched to the Garmin vivioaktiv 4s! Unfortunately I have no other suggestion. Turning it off didn't help with my watch either.