Hello oliverwide,
Thanks for reaching out to the Apple Support Communities. We understand that your Apple Watch Series 3 is not reporting the VO2 Max readings to your iPhone when you're using the Nike Run Club app.
Since this issue began after installing iOS 14 and watchOS 7, you’ll want to restart your iPhone and Apple Watch with the following sequence:
1 - Power off Apple Watch
2 - Power off iPhone
3 - Power on iPhone
4 - Power on Apple Watch
Be sure you've waited for each step to complete before proceeding to the next one. If you haven’t restarted the devices before, here’s how:
How to restart your Apple Watch
Restart your iPhone
If the issue persists after restarting both devices, you can contact Apple Support for more assistance: Get Support.