Action-based TabButton breaks TabBar behavior

import QtQuick 2.12
import QtQuick.Controls 2.12

Page {
    Action {
        id: a1
        text: "a1"
        onTriggered: console.warn("a1 triggered;", "tabBar.currentIndex:", tabBar.currentIndex)
    }
    Action {
        id: a2
        text: "a2"
        onTriggered: console.warn("a2 triggered;", "tabBar.currentIndex:", tabBar.currentIndex)
    }

    header: TabBar {
        id: tabBar
        TabButton {
            action: a1
        }
        TabButton {
            action: a2
        }
        onCurrentIndexChanged: console.warn("one would never see this message:", "currentIndex:", currentIndex)
    }
}