FR955 Heart rate reading errors during an activity.

You didn't mention what training or activity.

It's directly linked to my polar H10 training.

What activity did you start ? Running, strength, cardio?

I recall such HR issue when you start the wrong activity.

I chose the right activity, riding a stationary bike. in my opinion, regardless of the choice of activity, the heart rate should indicate what is the current one.

Are you using polar chest strap or watch optical HR?

Second @kevin  that would just be my problem and it was to seek assistance. shame, are many forums warning about it. I use polar h10 too.

I had a similar problem on a relative cold day. Otherwise the optic sensor did much better job than its predecessors.

That's the problem, it's on cold days. According to Kevin's support, he said it would be a one-off problem, but it's not. It may be that some regions of the world are not cold. Here in the south of Brazil yes. Cold days don't work the heart rate. Garmin-Kevin

See their answer:

From what I see in the main thread (https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-955-series/311787/fr955-heart-rate-reading-errors-during-an-activity) for that report, this looks to be (for now) isolated to you and one other user.  At this point, I'd encourage you to reach out to your local Garmin Product Support Team directly.

I was experiencing the same on my forerunner 945 with HRM pro and later on with a polar H10. When i did a labtest in a sportsmedical facility it turned out that i was suffering from afib. Tthat explained my bad readings. So for now hr doesn’t work for me. Don’t want to worry you. But test yourself in a medical facility if you can.