I have experienced the same thing or least something similar. I was just reviewing my steps for the past few days on the app. I know I reached over 10.000 steps but the app shows only 7,000. For the past four days I have lost at least 3,000 steps. I did sync. I watched it. I watched the numbers flicker back and forth, but the app showed my correct steps before I left the app. Any ideas?
I'm running iOS 10 and my app is up to date.