BusyIndicator doesn't spin in a modal popup

Steps to reproduce:
1. Create a new Flex mobile project with this application:

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark">

<fx:Declarations>
<s:SkinnablePopUpContainer id="myAlert" click="myAlert.close()">
<s:VGroup horizontalAlign="center">
<s:Label text="Loading..." />
<s:BusyIndicator />
</s:VGroup>
</s:SkinnablePopUpContainer>
</fx:Declarations>

<s:Button label="Start loading" click="myAlert.open(this, true);"/>

</s:Application>

2. Click on the button
3.

Actual Results:

The BusyIndicator doesn't spin

Expected Results:

The BusyIndicator should spin

Workaround (if any):