QSensor::active handling is buggy.

void QSensor::setActive(bool active)
{
    if (active == isActive())
        return;
    if (active)
        QTimer::singleShot(0, this, SLOT(start())); // delay ensures all properties have been set if using QML
    else
        stop();
}

Screen { // an Item with visible: false as default
	id: proxScreen
	onVisibleChanged: {
		console.log("proxScreen.visible: "+ visible)
	}

	ProximitySensor { //F09
		id: proximity
		active:proxScreen.visible
		onActiveChanged: {
			console.log("proximity.active: "+ active)
		}
	}
}

qml: proxScreen.visible: false
qml: proximity.active: false
qml: proximity.active: false