After a sync yesterday i lost a lot of data on my watch / app of that day.
All my steps, energy, ... of that day where lost in the connect app.
On my watch i still had the hearth-rate, body level, but those where not syncing anymore.
My activities where synced, but the calories and stept where lost.
I also lost my active minutes and so on.
I tried to reconnect my watch to the app, but without any luck.
Was is a drop of the wifi while syncing, not sure.
But i guess when the sync is not completed for any reason the sync should fail so you can try again later