Hello looch10,
Thanks for posting! We see that you're finding your activity doesn't appear to be syncing between your new iPhone 12 Pro and your Apple Watch. That's definitely important to look into, and we'd like to assist.
For verification, you see your activity on the Apple Watch, but it doesn't show up on the iPhone, correct?
As a first step, we recommend making sure both the iPhone and Apple Watch are fully updated:
Update your iPhone, iPad, or iPod touch
Update your Apple Watch
Also, you mentioned trying some troubleshooting. Did you already try to unpair and re-pair the devices? If not, see if that helps: Unpair and erase your Apple Watch
Also, make sure you've restarted both the iPhone and Apple Watch in that sequence:
Restart your iPhone
How to restart your Apple Watch
If you've tried these steps already or the issue persists after, use this link to Contact Apple Support to look into this further.
Kind regards.