VO2 Max stopped recording

I’ve just upgraded my series 3 Apple Watch for a series 6. I’ve set up from a backup, rather than a new watch. The reason I upgraded was because my series 3 kept recording an identical VO2 max every run. Whilst my runs were similar distance - 10km / 43-47 mins, the intensity was always varied.

I received my new series 6 earlier this week and have completed five runs, but no VO2 max is returned. I tried unpairing and repairing the watch after run three, but no change. Upgraded to WatchOS 7.0.1 yesterday and no VO2 today. I should add that all other activity, heart rate, Strava integration etc. is working as normal. The only issue is VO2 Max.

Why is this happening? Should I try and set up as a new watch? I was torn between buying a new Apple Watch and a Garmin and this has left me massively frustrated.