Heart rate during workout on watch 10

Hello,

when starting a workout, heart rate is very unreliable. It can be “in gray” and it can take as long as 20 minutes before having a real first heart rate. Sometime it will stay ok for the rest of the workout, but sometime it goes gray again.

Sometime, it won’t be grayed but will have a very too high measurement (like 170++ when I know I’m not even 130).

All the permission are granted, as “outdoor walking” is mostly OK, but as soon as I go “outdoor run”, it goes crazy.

Same behavior with built-in app or 3rd party like workoutdoor.

Note that i’m an apple watch user since series 4 and i’ve never had this issue before my new Watch 10, last september.

any idea? Thanks.