QVariantAnimation goes too fast when QQmlApplicationEngine shows window

The update interval of the QVariantAnimation goes too quickly when QVariantAnimation is used at the same time as a QQmlApplicationEngine.

Reproduction code attached.

Out of the box, the 10,000ms animation output ends something like this:

...
Current Time 9960 CurrentInterval 16 IntervalCount 623 IntervalTime 16
Current Time 9976 CurrentInterval 16 IntervalCount 624 IntervalTime 16
Current Time 9992 CurrentInterval 16 IntervalCount 625 IntervalTime 16
Current Time 10000 CurrentInterval 8 IntervalCount 626 IntervalTime 16
Elapsed 10013

However when the engine is allowed to load the QML (by uncommenting the engine.load() line), the animation goes too fast:

...
Current Time 9966 CurrentInterval 16 IntervalCount 598 IntervalTime *5*
Current Time 9983 CurrentInterval 17 IntervalCount 599 IntervalTime *5*
Current Time 10000 CurrentInterval 17 IntervalCount 600 IntervalTime *5*
Elapsed *3547*

You can see that the animation which should have taken 10,000 ms only took 3547. The currentInterval is the difference in values in the call to updateCurrentTime. This seems to be correct. What is not correct is the time between those calls - only 5ms! This means that the animation completes in around 30% of the time.

Note that if the root object of the QML is changed to something other than Window/ApplicationWindow e.g. Item, the animation rate returns to the correct ~16ms.

In addition, if I take out the QQmlApplicationEngine and replace it with a QWindow w and show it, the problem doesn't occur but when creating a QQuickWindow instead and showing it, the problem does occur.

Interestingly, publishing the type to QML and using it only there has the same issue:

qmlRegisterType<VariantAnimation>("VariantAnimation", 1, 0, "VariantAnimation");

import QtQuick 2.3
import QtQuick.Controls 1.2
import QtQuick.Window 2.2
import VariantAnimation 1.0

ApplicationWindow {
    id: window

    width: 300
    height: 100

    visible: true

    VariantAnimation {
        startValue: 0
        endValue: 1000
        duration: 10000

        property var now

        onStateChanged: if(state == VariantAnimation.Running) now = new Date()
        onFinished: console.log(new Date() - now)

        Component.onCompleted: start()
    }
}