SMP3 has a component that is responsible for storing the received user credentials (CSI). When you set the backend SSO to Basic, the user name and password are retrieved from there (CSI). When the AD password changed, and the user enters the new password when accessing SMP3, this new password will be passed on to NW backend.
As you can see, if the backend NW system is not aware of the changed password, the user won`t be able to log on. Therefore you have to take of the password sync or use a SSO solution.