Ember Component classNameBinding not binding

I have a component that is a button which needs to change it's classNames via a property of it parent component/controller:

// components/my-button/component.js
import Ember from 'ember';

export default Ember.Component.extend({
    tagName: 'button',
    classNameBindings: ['classNames'],
    // some other unrelated stuff following....
});

it's template:

// components/my-button/template.hbs
{{text}}
// nothing important here exept the test-output of:    
{{classNames}}

and I insert it like this in another component:

// component/parent-component/template.hbs
{{my-button
    classNames=variableClassNames
    text='foo'
}}


// components/parent-component/component.js
import Ember from 'ember';

export default Ember.Component.extend({

    isSortableDown: Ember.computed('track.sort', 'maxSort', function() {
        return this.get('track.sort')<this.get('maxSort');
    }),

    variableClassNames: Ember.computed('isSortableDown',function() {
        if(this.get('isSortableDown')) {
            return 'btn btn-xs btn-default';
        } else {
            return 'btn btn-xs btn-default push-20-r';
        }
    }),

    // other unrelated stuff...
});

Now here's my problem:
when isSortableDown is changing, variableClassNames in parent AND classNames in child (component/my-button) IS updated (also the test-output in my-button template).
BUT the classNameBindings are NOT updated, instead the classNames appear multiple times (when looking at the actual outputted DOM). Well, that's not 100% right, they do get added, but never removed.
So if the className push-20-r once gets added, it'll stay there (in the DOM), but never removed, even if the property classNames doesn't include it anymore.

Finally my question is if I'm doing something wrong,
or if the classNameBindings should not be updated (but why the name 'bindings' then??),
or if this is eventually a bug?

I'm on
Ember 2.0.1
jQuery 1.11.3

The only maybe relevant issues I found are:
http://ift.tt/1Z72Owa
http://ift.tt/1OM4Dng
but they don't have an answer... or are only party related

Sidenote: Yes, I want the component itself to be a button, not a div, because otherwise I'd have to change all the css.... I know I could do it by leaving the component a div and wrap that button and adjust it's classNames there.