title: ionic 4 angular route observable not working once I leave the view and re-enter
body: <p>I have a setup where in a single ionic 4 page, I am showing multiple items based on query params. Consider a list with different categories and onclick, I will update the query params. </p>
  
  <p>So I have an observable </p>
  
  <pre class="lang-js prettyprint-override"><code>this.route.queryParamMap.subscribe((data) =&gt; {
    console.error('in subscribe', data);
    this.updateCategories(data['params']);
  });
  </code></pre>
  
  <p>This works fine when I first come to page. But when I navigate to some other page and then come back, the <code>console.error</code> from this above code stops working. </p>
  
  <p>If I am correct, observables on route are self destroyed by angular (I also tried to unsubscribe on ngOnDestroy and then subscribe again). And when I come back again, it should subscribe to the route's queryParamMap and work as before.</p>
  
  <p>I am using ionic 4 beta 12 with Angular 6.0.8. Any ideas why this might be happening?</p>