By any chance are you using the app on IOS?
I had a similar issue with the Withings app not displaying some of the key metrics, such as BMI. Turned out that another app was rewriting my weight to Apple Health, which then got pulled into the Withings app — so every day I had 2 weight readings… one from the scale with all measurements, and one from the other app that just included weight.
Once I changed the perms of my other app to only read weight and not write it to Apple Health, the duplication stopped and my Withings app calcs and displays the metrics from the scale properly now.
Not sure if this is the same issue you are having, but might be worth checking.