See the attached example. CppObj is a QML_ELEMENT while CppBridge is a QML_SINGLETON.

The example creates 2 lists, each containing 100 instances of CppObj with JavaScriptOwnership but no parents:

• IDs 0 - 99 are stored in root.customProperty (a custom list<CppObj> property)
• IDs 100 - 199 are stored in listView.model

Code

import QtQuick
import QtQuick.Controls.Basic
import JSOwnershipStudy

ApplicationWindow 
    id: root
    width: 400
    height: 300
    visible: true

    header: Button {
        text: "Collect Garbage"
        onClicked: gc()
    }

    property list<CppObj> customProperty: CppBridge.multipleObjs(0)

    ListView {
        id: listView
        anchors.fill: parent
        model: CppBridge.multipleObjs(100)
        delegate: Label {
            required property CppObj modelData
            text: modelData.identifier
        }
    }
}

Steps to reproduce

1. Run the attached example
2. Click the "Collect Garbage" button
3. Close the window to quit
4. Repeat the test above, but scroll to the bottom of the ListView before doing Step #2

Outcomes

The CppObjs stored in customProperty (IDs 0 - 99) are never destroyed by the garbage collector but are always destroyed upon quitting.

However, for the CppObjs stored in ListView.model (IDs 100 - 199), their fate depends on whether a delegate is accessing the instance at the time when deletions are processed, and also on the Qt version: 

In other words, even though the returned lists are the same, the list stored in root.customProperty is ref-counted properly, but the list stored in listView.model is not. The exact manifestation is version-dependent.

Setting QV4_GC_TIMELIMIT=0 and QV4_MM_AGGRESSIVE_GC=1 does not help.