I have the same issue, except that mine is a 3-week old Series 3 Watch that's only been paired with a 8 Plus. In addition, the issue is not calibration nor tightness of watch nor Location Services, as the Workout app has all the information on my 21 minute walk today. All data is there, distance, heart rate, pace, etc. However, the Exercise ring in Activity refuses to add the 21 minutes. The distance via a walk is shown at the bottom of the Activity screens, if you scroll all the way down.
If I told it I did a walk for that time, why wouldn't it add it to the rings?
You implied in your first response that the workout had to be strong enough, brisk enough, to qualify. Where is that setting so I can turn it off? If I record a walk for a certain time & distance I don't need the watch to interpret whether it was exercise or not - *I* determine that.
My Fitbit Charge 2 has a choice for walk and it handles the data automatically with no fuss. I'm confused why the Apple Watch can't do the same.