this.platform.backButton.subscribeWithPriority not called ionic 5

subscribeWithPriority not called for Ionic 5. This is app.component.ts function.

initializeApp() {
    this.platform.ready().then(() => {
      this.statusBar.styleDefault();
      this.splashScreen.hide();
      alert("initialed!"); // this called, and I can see this.
      this.platform.backButton.subscribeWithPriority(0, (processNexthandler) => {
        console.log('Back press handler!');
        alert(888) // this never called
      });
   }
}

The first alert shows that it call initializeApp function. But I never see next alert 888 I tried other priorities, like 9999 or 0, -1, all not working.

Thanks