Unit testing service in EmberJS: How to prevent interaction between tests?

I'm seeing a strange interaction when writing unit tests for services in ember.js. It only occurs on arrays.

I'm having a service which has an array as property. The state of array is shared between different tests. I don't see this behavior if property has a simple type (e.g. number). Do you have any hint what's going on?

Here is a ember-twiddle demonstrating the issue: http://ift.tt/2jJQe64