Why is my Apple Watch Heart Rate feature greyed out?

I have an apple watch 3. My heart rate used to be displayed during workouts, but now it does not. I have made sure that heart rate is enabled during those activities but it's greyed out (when I go to the display view on my phone in workout view), and can’t be changed. It's not happening for all exercise types but for the most common ones I use, outdoor running and walking. I’ve followed all the suggested setting changes, i wear the watch snugly, and have no wrist tattoos. It used to work; and I had no trouble when I used to use strava, but then it was removed from my watch and for the last two weeks the above issue has been happening.

[Re-Titled by Moderator]