Why do I need to reload my page to get data from a subjectSubscription in Angular?

I'm running into a problem in my angular app.

I want to get data from firebase in my component "statsview" using ngOnInit() and a subscription to a subject.

Here is the subscription to the subject.

      export class StatsviewComponent implements OnInit, OnDestroy {
      
      stats : Stats;
      statsSubscription : Subscription;  
      
      constructor(private hbService:HbService) { }
      
      ngOnInit(): void {
        this.statsSubscription = this.hbService.statsSubject.subscribe(
          (stats:Stats) => {            
              this.stats= stats;           
          }
        );
        this.hbService.statsSubject.next(this.stats);
      }

Here is what I want to display.

      
      <h1>nc : {{ stats.nc }}</h1>
      <h1>Réponse au 1er Message : {{ stats.firstMess }}</h1>
      <h1>Date : {{ stats.firstDate }}</h1>
      <h1>FC : {{ stats.fc }} </h1>

When the application loads the page the value "stats" from the observable gets the right data and it works BUT when I go on an other component and get back to my "statsview" the value "stats" is undefined and nothing appears in the view. To get my statsview working I need to reload the page (F5)...

I don't understand why it does not work correctly when I navigate in my application and go back to my statview without refreshing the page.

My subject declaration is in hbService and I put the function getStats() in the constructor.

Here is the function getStats() in hbService.

getStats(){
    firebase.database().ref('/stats/').on('value', (data: DataSnapshot) => {
      this.stats = data.val() ? data.val() : this.stats;
      this.statsSubject.next(this.stats); 
    });

Here is the function called in the constructor of hbService and the declaration of the subject.

export class HbService {

  stats : Stats;
  statsSubject = new Subject<Stats>();
   
  constructor(private router: Router) { this.getHbs(); this.getStats(); }

I wanna know why when I navigate in other component and get back to my component statsview my observable does not get data from the subject subscription.