With 6.8.2, the following example generates a warning about a binding loop, and item1 is not assigned:

import QtQuick
import QtQuick.Controls

ApplicationWindow {
    header: Item {
        property Item item1: item1
    }

    Item { id: item1 }
} 

If the QML compiler is disabled, the issue does not occur. Removing `header:` (so the first Item is not assigned to any property) also avoids the issue. The issue did not occur with Qt 6.6.1.