You should change your IsLoggedGuard like this:

@Injectable()
export class IsLoggedGuard implements CanActivate {

  constructor(private auth: AuthService, private router: Router) { }

  canActivate() {
    return this.auth.user
                    .take(1)
                    .map(authState => {
                       if (authState) {
                          //user is already loggedin
                          //route the user to Dashboard page
                          //Or a page where you want the app to naviagte
                          this.router.navigate("dashboard route");
                          //dont show the Login page
                          return false;
                       } else {
                         //user is not loggedin
                         return true;
                       }
                    });
  }

}

You were seeing the issue because when you enter the "localhost:4200/#/auth" url in the browser, then your AuthGuard.user.subscribe [i.e. in the constructor this.user.subscribe(] might not have emitted any value yet when IsLoggedGuard's canActivate() executed [i.e. AuthService.isLoggedIn() may return false because subscribe callback might not have executed (which fills the userDetails)].

Let me know if it solves your problem.

There could be a better way to implement your AuthService as well as Guards to make use of AuthService. Let me know if you want a better code.

EDIT - Another approach to write AuthService

Let's change the AuthService like this:

export class AuthService {

    //NOTE: I AM JUST SHOWING TWO THINGS - isUserLoggedIn AND userDetails
    //FROM THIS CODE YOU WILL GET AN IDEA HOW TO WRITE OTHER PROPERTIES WHICH ARE RELEVANT FOR YOUR APP

    //This will be used as a source for various observables
    private _authState$: Observable<any>;

    //Have an observable which will tell if user is loggedin or not
    isUserLoggedIn$: Observable<boolean>;
    userDetails$: Observable<firebase.User>;

    public userProfileRef: firebase.database.Reference;

    constructor(private _firebaseAuth: AngularFireAuth, private router: Router) {            
      this.userProfileRef = firebase.database().ref('/userProfile');
      this.setupObserables();
    }

    setupObserables() {

        // this observable will broadcast the emited values to multiple subscribers [or composed/dependent observables]
        this._authState$ = this._firebaseAuth.authState
                                        .publishReplay(1)
                                        .refCount();

        // lets componse/derive different observables required by the consumer of this service

        // This observable's emitted value will tell if user is logged in or not
        this.isUserLoggedIn$ = this._authState$
                                   .map(user => {
                                        return user ? true : false;
                                    });

        // This observable's emited value will return the user's detail [NOTE If user is not logged in then the emitted value will be NULL
        // i.e. userDetail is NULL; Your consumer of this observable should decide what to do with NULL/NOT NULL Value]        
        this.userDetails$ = this._authState$
                                .map(user => user);
    }    

    doSignOut() {
      this._firebaseAuth.auth.signOut()
        .then((res) => this.router.navigate(['/auth/login']));
    }
  }

Now let's make use of updated AuthService in IsLoggedGuard:

    @Injectable()
    export class IsLoggedGuard implements CanActivate {

      constructor(private auth: AuthService, private router: Router) { }

      canActivate() {
        return this.auth.isUserLoggedIn$
                        .take(1)
                        .map(isLoggedIn => {
                           if (isLoggedIn) {
                              //user is already loggedin
                              //route the user to Dashboard page
                              //Or a page where you want the app to naviagte
                              this.router.navigate("dashboard route");
                              //dont show the Login page
                              return false;
                           } else {
                             //user is not loggedin
                             return true;
                           }
                        });
      }

    }

Now let's make use of updated AuthService in AuthGuard:

    @Injectable()
    export class AuthGuard implements CanActivate {

    constructor(private auth: AuthService, private router: Router) { }

    canActivate() {
        return this.auth.isUserLoggedIn$
                   .take(1)
                   .map(isLoggedIn => {
                    if (!isLoggedIn) {
                       //user isNOT loggedin
                       //route the user to login page
                       this.router.navigate(['auth/sigin']);
                       //dont show the next route
                       //lets fail the guard
                       return false;
                    } else {
                      //user is loggedin; pass the guard i.e. show the next route associated with this guard
                      return true;
                    }
                 });
        }

    }

Now suppose some component (assume the component name is UserComponent) you want to show the logged in user detail:

....component decorator...
export class UserComponent implements OnInit {

    userDetails$: Observable<User>;
    constructor(private _authService: AuthService) {
        this.userDetails$ = this._authService.userDetails$;
    }
}

Render the userDetails like this:

<div *ngIf="(userDetails$ | async) as userDetails">
    <!-- Render your user details here -->
    <!-- If userDetails is NULL then nothing will be rendered -->
</div>

THINGS TO NOTE - In this updated code NOWHERE we are subscribing to any of the observables. Notice async in the component template, this takes care of subscribing/unsubscribing of the used observable.

Hope it will give you a direction/idea. Let's be "Reactive" as much as possible instead of "Imperative"..:)

NOTE: I have tested the equivalent code in rxjs6. Looks like you are using rxjs5 so I have adjusted the posted code as per rxjs5. Hope it will work.