When a proxy model's source model is set after initializiation, QML views show duplicate but undefined items

That's the issue:

A Repeater / ListView, that uses a QAbstractProxyModel's derived model, renders twice as many rows as the model's rowCount() indicates. See attached screenshot.

The issue happens, when the proxy model's sourceModel is set imperatively after initialization.

The issue does not happen, when the proxy model's sourceModel property is set declaratively.

QMLTestProxyModel {
    id: qmlTestProxyModel

    // Issue does not happen, when sourceModel is set during initialization
    //sourceModel: qmlTestModel

    // Issue happens, when sourceModel is set imperatively, after initialization
    Component.onCompleted: {
        qmlTestProxyModel.sourceModel = qmlTestModel
    }
}

Please, see attached example project.