at startup system preferences-security & privacy screen opens

Go to System Preferences > User & Groups > Startup Items and check there. If it is selected as a Startup Item use the (-) minus button to delete it.

Also check under Apple menu Shutdown, Restart or Logout that "Reopen windows when logging back in" is NOT check marked.

Does this happen if you startup in Safe Mode?

Use safe mode to isolate issues with your Mac - Apple Support

Also, if this is a new Mac then you have 90 days of free telephone support. (800) MYAPPLE. I suggest you use it to open a ticket. You also have 14 days to return it for a full refund.