QIcon not using Hi DPI pixmap version

On my Linux/Wayland (reproduces on Windows 10 as well) with DPI scaling set to 200%, QIcon does not automatically use the `@2x` Hi DPI pixmaps.

I have created a simple reproducer to demo the issue. It contains a QListView widget in icon mode with an icon size of 64x64. Attached to the widget is a simple model with the following  `data()` method:

QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override {
    if (role == Qt::DisplayRole) return "Hi-dpi repro";
    if (role == Qt::DecorationRole) return QIcon(":/icon64.png");
    return {};
};

In the resources there are two icons: `icon64.png` which is 64x64 pixels and `icon64@2x.png` which is 128x128 pixels.

I would expect the 128x128 version to be used. Actually the 64x64 version in shown, resulting in poor image quality.

When porting the attached reproducer to Qt5 and enabling `AA_UseHighDpiPixmaps`, the issue disappears. So it seems to me this is a Qt6 regression.