Wipe doesn't transition alpha values

Steps to reproduce:
1. Compile and run:

<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark" >
<s:states>
<s:State name="normal" />
<s:State name="wipe" />
</s:states>

<s:transitions>
<s:Transition fromState="normal" toState="wipe">
<s:Wipe duration="2000" direction="left" targets="

" />
</s:Transition>
</s:transitions>

<s:controlBarContent>
<s:Button label="change state" click="currentState=currentState=='wipe'?'normal':'wipe'"/>
</s:controlBarContent>

<s:layout>
<s:VerticalLayout />
</s:layout>

<s:Button id="content1" label="content1"
width="100" height="100"
alpha="0.5" alpha.wipe="1" />

<s:Button id="content2" label="content2"
width="100" height="100"
alpha="0" alpha.wipe="1" />

</s:Application>

2. Click change state
3.

Actual Results:

The Wipe transition doesn't appear to happen.

Expected Results:

The Wipe transition should happen.

Workaround (if any):