This is expected behaviour, generally, if Portal for ArcGIS and ArcGIS Server are configured to use the web-tier authentication. Portal for ArcGIS and ArcGIS Server can be configured to use the web-tier authentication with Integrated Windows Authentication (IWA) with Microsoft Windows Active Directory, to allow web single sign-on experience. Therefore, when using IWA, logins are managed through Microsoft Windows Active Directory, and there is no need to sign in and out of the portal and server. Instead, when opening the website, users are signed in using the same account used to log in to Windows.
Portal for ArcGIS can also be configured to use a Security Assertion Markup Language (SAML)-compliant identity provider and ArcGIS Account logins. If the log-in from an identity provider account is selected, the user is automatically logged in with web-tier authentication and the logout button is not available. However, if ArcGIS account logins are disabled, the user is automatically logged in using the Enterprise logins, and the option to choose is no longer prompted.
If Portal for ArcGIS is federated with ArcGIS Server and is configured to use web-tier authentication, the same behavior also automatically occurs on ArcGIS Server. When Portal for ArcGIS is federated to ArcGIS Server, the security settings of ArcGIS Server are taken over by Portal for ArcGIS. Hence, logging in is no longer required and the logout button is absent from the server page.