authentication.getPrincipal() is always String instead of UserDetails

Question

In my WebSecurityConfigurerAdapter I have this:

@Bean
public DaoAuthenticationProvider daoAuthenticationProvider() {
    DaoAuthenticationProvider daoAuthenticationProvider = new DaoAuthenticationProvider();
    daoAuthenticationProvider.setPasswordEncoder(passwordEncoder());
    daoAuthenticationProvider.setUserDetailsService(this.userDetailsService);
    return daoAuthenticationProvider;
}

and the following in the UserDetailsService:

@Service
public class AppUserDetailsService implements UserDetailsService {

    @Override
    public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {

        AppUserEntity appUserEntity = this.appUserRepository.findByUsername(username).orElseThrow(() ->
            new ResourceNotFoundException("Current user not found."));

        return new AppUserDetails(appUserEntity);
    }

}

However, I cannot fetch my AppUserDetails implements UserDetails object with

Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
// Throws exception because the principal is a String
AppUserDetails appUserDetails = (AppUserDetails) authentication.getPrincipal();

because authentication.getPrincipal() always returns just a String (in this case the user's username). However, I expected it to return a AppUserDetails. Why is this not the case and how can I change this?

---

I have tried

SecurityContextHolder.getContext().getAuthentication().getDetails()

but this returns OAuth2AuthenticationDetails and not the UserDetails I was hoping for.

---

Trying this won't work either. The returned details object is null.

OAuth2Authentication oAuth2Authentication =
                (OAuth2Authentication) SecurityContextHolder.getContext().getAuthentication();

Authentication userAuthentication = oAuth2Authentication.getUserAuthentication();

Object details = userAuthentication.getDetails();