Element is not decorated on dynamic observables

Please refers to http://jsbin.com/mudeko/4/edit?html,js,output for a live demo

Steps to recreate:
- Click on Add Activity Type
- Click on Save

the form is invalid but the element is not decorated.

However, this is working on the initial binding:
- Reload the page
- Click on Save

the form is invalid and the element is decorated.

Seems that the binding is detached on the following reassignments.

Start with the JSBin demo linked in the issue and reproduce the dynamic flow: click Add Activity Type, then Save, and compare it with the initial-binding flow after reload. Trace why validation is detached after the reassignment; done means the invalid dynamically added element is decorated as it is during initial binding.

Written by the indexing model from the issue text.