Undesirable scope bleed between components in Ember

I have a component [ui-button] (github) where I'm adding a wrapping component called ui-buttons (demo here). The problem is that the wrapping component seems to receive registrations not only from its children but from ALL children that are on the page! Effectively this property is acting like a static variable across all instances of ui-buttons. I didn't even know you could do this and in this case its definitely an undesirable effect.

In the demo link above try clicking on the the "disable the group" button and notice that it disables ALL buttons. So what am I doing?

Structurally it looks like this:

{{#ui-buttons as |group|}}
  {{ui-radio-button title='foo' group=group}}
  {{ui-radio-button title='bar' group=group}}
  {{ui-radio-button title='baz' group=group}}
{{/ui-buttons}}

In this process I have child elements (e.g., ui-radio-button) register themselves with ui-buttons. The item-level registration code is:

_registration: on('init', function() {
  const group = this.get('group');
  if(group) {
    group._registerItem(this);
  }
}),

and the group-level registration code is:

_registerItem: function(child) {
  console.log('registering %o with %o', child, this.get('elementId'), this.get('_registeredItems.length'));
  this.get('_registeredItems').pushObject(child);
},

If you note the group-level registration has a "console.log" statement and that produces very encouraging results (it recognizes the element ID of the group as being distinct) alongside worrisome results (the registry "length" continues to grow across all):

I suspect this is down to async or this complexities but I'm now at a loss on how to proceed.