Why is my Ember.js controller not reacting to observed bound property on a controller associated via needs?

I'm building a nested layout where some information can be set on a controller that controls the mode of the application, affecting the controllers nested under it.

I feel like I've tried everything, but I can't get the nested controllers (and therefore components) to respond to the changes in a controller further up in the route hierarchy, despite observing its property.

Below is a diagram of the controller/component setup and some simplified code to show what I'm seeing.

The component that triggers the event has the following JS:

App.LimitTriggerComponent = Ember.Component.extend({
  actions: {
    selectGroup: function( groupId, groupType ){
      this.sendAction( "action", groupId, groupType );
    }
  }
});

and the following handlebars:

<div>
  <span {{action "selectGroup" 1 "Group Type"}}>
    Some Text
  </span>
  <span {{action "selectGroup" 2 "Group Type"}}>
    Some Other Text
  </span>
</div>

And below is my controller setup:

App.NamespaceController = Ember.Controller.extend({
  activeGroup: { id: null, type: null },

  actions: {
    limitByGroup: function( groupId, groupType ){
      // clicks are responded to in this handler
      // inspecting the activegroup here yields the expected results
      this.set( "activeGroup", { id: groupId, type: groupType });
      // also here, the property is actually updated, but observers aren't fired
    }
  }
});

App.NamespaceInnerController = Ember.Controller.extend({
  needs: ["namespace"],
  activeGroup:  Ember.computed.alias( "controllers.namespace.activeGroup" ),

  init: function(){
    // this logs as expected, although it's an object literal, not an Ember object
    console.log( this.get( "activeGroup" ));
  },

  testFn: function(){ 
    console.log( "Really baffled as to why this won't log anything." ); 
  }.observes( "activeGroup" )
});

I doubt it makes sense to be exhaustive about everything I've tried, and at this point I'm kind of wondering if it could be a load order issue or something else seemingly-orthogonal. I'd be open to any suggestions at all after fighting with this for a long time :/

Thanks!