I made a Kirigami.OverlaySheet sheet on which there is a Controls.ComboBox.
The popup menu of Controls.ComboBox is shown under Kirigami.OverlaySheet.

my system: linux box, artix distro, kirigami2 version 5.39.0-1
Code sample:

Kirigami.OverlaySheet
    {
    id: settings
    ColumnLayout
      {
      Controls.Label
        {
        text: qsTr("Settings")
        Layout.alignment: Qt.AlignHCenter
        }

      RowLayout
        {
        Controls.Label
          {
          text: qsTr("Choose one: ")
          }
        Controls.ComboBox
          {
          Kirigami.Theme.inherit: true
          currentIndex: 0
          model: ["one", "two", "three", "four"]
//          onCurrentIndexChanged: console.debug(currentIndex)
          }
        }


The popup menu (one, two, three, four) is under the OverlaySheet (and it's very difficult to select it)