Why subject isn't working inside SwitchMap?

Question

I set up a service to make an http Request.

    private readonly changeSelectedId$ = new Subject<string>();
    
    private readonly selectedId$ = this.changeSelectedId$.pipe(
    distinctUntilChanged(),
    untilDestroyed(this),
    switchMap((id) => this.httpClient.get<AuthenticatedSubject>(`${this.endpointService.get()}/${id}`)),
    catchError((e) => {
        return EMPTY;
      })
  );

    public getNewData(newId: string): Observable<AuthenticatedSubject> {
    this.changeSelectedId$.next(newId);
    return this.selectedId$;
    }

In my component i listen for emission from a formControl and i use switchMap operator to call the service and, at last, i subscribe to get the result:

ngOnInit(): void {
    this.formControl.valueChanges.pipe(
      untilDestroyed(this),
      switchMap(newId=> this.loginDataAccessService.getNewData(newId)))
      .subscribe(authData => console.log(authData))
  }

The problem is that, if I initialize changeSelectedId as Subject, it doesn't work. Instead, using ReplaySubject, it works as expected and i get the value in the console. I know the difference between Subject and ReplaySubject, but it's not clear why, using Subject, it doesn't work. Thanks to who will answer me.