No idea, but it seems to work? (#2526)

src/layouts/app/home-assistant.js

@@ -50,11 +50,13 @@ export class HomeAssistant extends ext(PolymerElement, [
         route="[[route]]"
         pattern="/:panel"
         data="{{routeData}}"
+        tail="{{subroute}}"
       ></app-route>
       <template is="dom-if" if="[[showMain]]" restamp>
         <home-assistant-main
           hass="[[hass]]"
           route="[[route]]"
+          tail="[[subroute]]"
         ></home-assistant-main>
       </template>
 

src/layouts/home-assistant-main.js

@@ -81,6 +81,9 @@ class HomeAssistantMain extends NavigateMixin(EventsMixin(PolymerElement)) {
     return {
       hass: Object,
       narrow: Boolean,
+      tail: {
+        type: Object,
+      },
       route: {
         type: Object,
         observer: "_routeChanged",
@@ -135,7 +138,7 @@ class HomeAssistantMain extends NavigateMixin(EventsMixin(PolymerElement)) {
 
   connectedCallback() {
     super.connectedCallback();
-    if (this.route.prefix === "") {
+    if (this.tail.prefix === "") {
       this.navigate(`/${localStorage.defaultPage || DEFAULT_PANEL}`, true);
     }
   }