DITA attribute ignored in FM11

I'm having a dip into DITA with FrameMaker 11 (because that's the version I currently have).

Here's a sample topic in which I've defined a product attribute for two paragraphs:

<reference id = "id18AC90HG0CJ">
<title>test</title>
<shortdesc>Product attribute demo</shortdesc>
<refbody>
<section>
<title>Supported file formats</title>
<p product="A">JPEG</p>
<p product="B">EPS</p>
</section>
</refbody>
</reference>

When I save and reopen the topic file, the attribute values appear in XML view but not in WYSIWYG view, neither in structure view nor in the attributes pane.

Also, when I print a ditamap and apply a ditaval filter on the product attribute, I get the unfiltered output, which probably means the product attribute is ignored as well.

<?xml version="1.0" encoding="UTF-8"?>
<!-- filter.ditaval -->
<val>
    <prop att="product" val="A" action="include"/>
    <prop att="product" val="B" action="exclude"/>
</val>

Has this been a known bug in FM 11, or is something wrong with my configuration?