After Samsung Galaxy Watch 42mm with One UI update, I can't install Samsung health update on the watch. Watch just disconnects when update is installing. I have Samsung Galaxy S9 (One UI) & Samsung Galaxy Watch 42mm (One UI).
Same here with 46mm and OnePlus phone
edit: unlike others I was able to install another app, but the installation of the Health app still ends in loosing connection with the watch and a botched install. So it has something to do with this specific app, so it seems - or at least with an app that has a close relation to the connection between watch and phone.
Galaxy S8+ (One UI) and Watch 46 mm (One UI) here.
Had the same problem as you. Fixed by
1. Update Samsung Health on Watch and let it disconnect.
2. Reconnect manually and make a backup of Watch to account.
3. Reset Watch, put backup back.
4. Samsung Health is now updated on connected Watch.
Factory reset ...