Given the following code (untested, for illustration):

Item {
    id: root
    property int x: 0
    NumberAnimation {
        id: anim
        target: root
        property: "x"
        from: x
        to: x + 10
        duration: 1000
        loops: Animation.Infinite
    }
}  

Now, anim.from is initialized to 0 and anim.to to 10 in every iteration of the loop. This means that x will go from 0 to 10, and then restart from 0 to 10, ad infinitum. This behaviour won't change in qml, but it should be documented (in http://doc.qt.io/qt-5/qml-qtquick-numberanimation.html).

The workaround is to replace

        loops: Animation.Infinite

with

        onStopped: start()