i had the same thing a few days ago.
Logged in again and all old data was lost.
I replaced my running system with a backup i did some days before this happened and started uhealth again.
I see some old data again in the app but then there was a little message displayed that i have to log in again and everything was lost.
I don't know how to go on.
Hopefully some of the other users are interested and can help us with their support.
Today Uhealth asked to log in after all the updates (current 4.0.247 ST) and found all data from 2016 were lost
This month steps were only recorded from the 4th day and one day from september with inaccurate data. I could login to Lenovo account but it didn't bring anything back.