Without logging into the instance, while opening the service portal home page url as below,
User expect to get the login page configured in the portal record, but user is directly presented with the home page instead of login page.
Any Supported Release.
Homepage (index) page is set to pubic as shown in the below screenshot and hence system bypass login page and displays homepage.
Do not set the homepage (index page) as public, so that user will be provided with login page.