WebP: animations don't respect alpha

#include <QtWidgets>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QLabel label;
    label.resize(300, 300);
    label.setAlignment(Qt::AlignCenter);
    label.setMovie(new QMovie("path/to/busyindicator.webp"));
    label.movie()->start();
    label.show();

    return app.exec();
}

import QtQuick 2.9
import QtQuick.Window 2.2

Window {
    width: 300
    height: 300
    visible: true

    AnimatedImage {
        anchors.centerIn: parent
        source: "file:///path/to/busyindicator.webp"
    }
}