Heart with circle arrow in middle

I’d guess that’s the “retry” icon.

The sensor can have difficulty reading data due to sweat, fit, cold temperatures, tattoos, perfusion, and likely other factors.

For fit and use, see > Get the most accurate measurements using your Apple Watch - Apple Support

Is that under the Current heading? Can you provide a screenshot of it? When it happens again, press digital crown and then the side button. If you have Siri enabled by digital crown, disable that first.

Many people have asked about this... but it looks like the jury is still out. Here's the basic detail of that feature:

Check your heart rate on Apple Watch

Thanks