If elements of a Column are created via Loader, the loader sourceItem has to be defined inside the Loader scope. If it is defined outside, via a property, index and other model properties are not accessible inside the delegate.
See the attached example. Working version is commented.
The working version gives no possibility to define the delegate outside of the component.

UPD:
I found in description that it was deliberately made to work this way.
The documentation misses description how to:
1) pass through all properties of QAbstractListModel if they can't be explicitly defined inside Loader. If I have, for instance, a rolename "node "defined in the model, when I try to access it inside the delegate, it shows "ReferenceError: node is not defined"
2) manually set Loader as the context object for the component it is loading.