I got a notification on Wednesday about a new challenge from my Apple Watch activity app and as I looked at it, I realized that I had no data prior to getting a new phone at the beginning of November.
It turns out that the data isn’t connected to your iCloud account and can only be added back to your phone by restoring from an encrypted backup.
I exported the last two months of activity from my phone and saved it to my iCloud Drive. Then I followed the steps here to restore all my Health data since May 2015.
Lastly I used this app to import the last two months of data back in.
It’s all there now, but I’m missing almost two months of rings in my Activity app. Definitely better than losing the 18 months of data.