Enzo,
Both the on change and not found microflows will not trigger due to an error in the widget.
Running the widget in firefox with firebug you'll see an error being thrown when entering a non-existing value:
An error occurred while handling queued requests: obj.save is not a function
When a non existing value is entered the widget tries to find the object, as this can't be found the field is cleared. This seems logical as the value entered can't be stored in the association table and should therefore be empty.
For the issue with the non-existing objects I would file a ticket with support.