I have a fresh install of Sync. On startup, Sync main window pops-up. It should stay minimized.
Sync version and system OS details attached.
The key was already set to MINIMIZED for me and I was still having the same issue. The following did work, however.
After the Fall Creator’s Update, Windows automatically reopens apps that were open upon restart or shutdown. This includes Sync. To disable this, navigate to the following location:
Settings > Accounts > Sign-In Options
Scroll down to Privacy on the right and then set the following to Off:
Use my sign-in info to automatically finish setting up my device