I managed to do this but it is not easy and may take you a few tries.
You not only loose the app data but you have to set your phone up as new not from a backup. So think about this before you try. And there is no guarantee that it will work. I found it by trying.
1) Unpair your watch.
2) Go to setting/your name/iCloud/health and turn off and delete. (If it is turned on)
3) In the health app remove the watch as a source and delete all it’s data
4) Setting /general/iPhone storage
find the watch app and at the bottom delete all backups
5) Setting/general/iPhone storage
find the activity app and delete the app and all data
6) Hard reset the phone
7) Start the pairing of the watch up to the point after you get a few screens in, no need to go too far in hit the back button and it will tell you it is cancelling the pairing. Say ok
8) Repeat steps 4 (if there’s any backup made) and step 6 & 7
9) Go to setting/your name/iCloud /iCloud backup and back up now
10) Now pair your watch and set up as new
I had to do these steps 2 or times in order to clear everything. Omitting step 9.
One you’re happy with the results go to setting/your name/iCloud/health and turn on health for the cloud (if you had it turned on at the beginning) and redo step 9. Then make sure you watch shows as a source in the health app you may need to set it’sthe priority as well
When you pair your watch one of the steps the install does is download the activity app so you don’t have to. Let the install do it.
Like i I said this is what worked for me. It may or may not work for you.