Hi NewPhone11Pro,
Welcome to Apple Support Communities. We understand that you're missing Activity data, after pairing your Apple Watch to a new iPhone. We'll be happy to help.
We recommend to see if the Activity data is displaying a progress bar, indicating that it is restoring your data from iCloud. To verify that, you can open the Activity app on your iPhone, tap on History and see if, "Restoring Activity data from iCloud" appears.
In addition, you'll want to see if, "Health" is turned on for iCloud. On your iPhone, please go to Settings > your name at the top > iCloud > Health.
We look forward to hearing back from you.
Kind regards.