Tabbing to different radio button groups doesn't happen when the groups aren't defined on the application

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"
xmlns:local="*">

<fx:Declarations>
<fx:Component className="ActivityRadioButtonGroup">
<s:VGroup>
<fx:Declarations>
<s:RadioButtonGroup id="buttonGroup"/>
</fx:Declarations>
<s:RadioButton id="radio1" value="1" label="1" group="

{buttonGroup}" />
<s:RadioButton id="radio2" value="2" label="2" group="{buttonGroup}

" />
<s:RadioButton id="radio3" value="3" label="3" group="

{buttonGroup}" />
<s:RadioButton id="radio4" value="4" label="4" group="{buttonGroup}

" />
</s:VGroup>
</fx:Component>
</fx:Declarations>

<s:VGroup gap="20">
<local:ActivityRadioButtonGroup />
<local:ActivityRadioButtonGroup />
</s:VGroup>

</s:Application>

2. Give focus to the application
3. Try to tab to the second radio button group

Actual Results:

Unable to tab to the second radio button group

Expected Results:

You should be able to tab to the second radio button group

Workaround (if any):

Define the RadioButtonGroups at an Application level:

<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:local="*">

<fx:Declarations>
<s:RadioButtonGroup id="group1" />
<s:RadioButtonGroup id="group2" />

<fx:Component className="ActivityRadioButtonGroup2">
<s:VGroup>
<fx:Script>
<![CDATA[
import spark.components.RadioButtonGroup;
[Bindable] public var rbg:RadioButtonGroup;
]]
>
</fx:Script>
<s:RadioButton id="radio1" value="1" label="1" group="

{rbg}" />
<s:RadioButton id="radio2" value="2" label="2" group="{rbg}

" />
<s:RadioButton id="radio3" value="3" label="3" group="

{rbg}" />
<s:RadioButton id="radio4" value="4" label="4" group="{rbg}

" />
</s:VGroup>
</fx:Component>
</fx:Declarations>

<s:VGroup gap="20">
<local:ActivityRadioButtonGroup2 rbg="

{group1}

" />
<local:ActivityRadioButtonGroup2 rbg="

{group2}

" />
</s:VGroup>

</s:Application>