Mutuable property passed from template is not updated

I have a case where I am passing a property to child component wrapped inside a mut helper. Then I am updating this value in child component, and send action into parent component using closure action.

Then if I read this value in parent component with get, I get correct value. But if I pass this property to action from template, I get not updated value.

I've provided an Ember-Twiddle to illustrate this problem (check out console.log).

Am I missing something in this solution? Or is it working as expected? Please take a look at onInnerUp action in index controller to see what I am trying to do.

I know this is very messy description, hope someone will understand what am I struggling with