It has been confirmed that this issue was a bug in version 11.1 and has been resolved in version 11.2 (Reference: BUG-000165238)
When ArcGIS Enterprise Portal is configured with SAML (Azure AD) and in Azure the Logout URL is set, upon signing out users will get a server error 404 – File or directory is not found instead of the Portal Sign Out page
Workaround solution is to leave the logout URL blank in Azure AD.