SOLVED
Same issue with new iPhone 16 with iOS 18. I noticed my iPad has all my health data on it, so After trying everything I found on line to no avail I finally decided to go for it, and IT worked.
On your iPhone:
Go to settings.
Click on your account name at the top.
Click on iCloud.
Click on “see all” under the Saved To Cloud section.
Click on Health.
Turn off the “ Sync this iPhone “ toggle
EXIT out and shut down your iPhone and your Apple Watch.
wait 10 seconds then turn iPhone back on.
go to the same section and toggle the “Sync this iPhone” to On.
it takes about 15 minutes to download your health data from the cloud.
turn on your Apple watch now if you have.
after it’s done downloading, check your data. It should all be there. At least it worked for moi. Aloha🤙🏼