Firebase onDisconnect not always working, users are still online

I'm trying to display all users connected or "away" of my app.

90% of the time it works fine but sometimes they are users that leaves and they are still marked as online, I can't find why. As far as I know desktop app works better than mobile (but it can be wrong on desktop)

Here is the code of the service:

export class PresenceService {

  constructor(private afAuth: AngularFireAuth, private db: AngularFireDatabase) {
    this.updateOnUser().subscribe();
    this.updateOnDisconnect().subscribe();
    this.updateOnAway();
  }

  getPresence(uid: string) {
    return this.db.object(`status/${uid}`).valueChanges() as Observable<any>;
  }

  getUser() {
    return this.afAuth.authState.pipe(first()).toPromise();
  }

  async setPresence(status: string) {
    const user = await this.getUser();
    if (user) {
      return this.db.object(`status/${user.uid}`).update({ status, timestamp: this.timestamp, uid: user.uid });
    }
  }

  get timestamp() {
    return firebase.database.ServerValue.TIMESTAMP;
  }

  // Updates status when logged-in connection to Firebase starts
  updateOnUser() {
    const connection = this.db.object('.info/connected').valueChanges().pipe(
      map(connected => connected ? 'online' : 'offline')
    );

    return this.afAuth.authState.pipe(
      switchMap(user => user ? connection : of('offline')),
      tap(status => this.setPresence(status))
    );
  }

  updateOnDisconnect() {
    return this.afAuth.authState.pipe(
      tap(user => {
        if (user) {
          this.db.object(`status/${user.uid}`).query.ref.onDisconnect()
            .update({
              status: 'offline',
              timestamp: this.timestamp,
              uid: user.uid
            });
        }
      })
    );
  }

  async signOut() {
    await this.setPresence('offline');
  }

  // User navigates to a new tab
  updateOnAway() {
    document.onvisibilitychange = (e) => {
      if (document.visibilityState === 'hidden') {
        this.setPresence('away');
      } else {
        this.setPresence('online');
      }
    };
  }
}