DITA VAL Filtering does not work on ditamaps that are referenced as topics

I have two DITA maps.  One is the top level.

<map xmlns:ditaarch="http://dita.oasis-open.org/architecture/2005/">
<title>Main DITA Map</title>
<topicref href="conceptA.xml" type="concept" product="A">
<topicmeta><navtitle>Concept A</navtitle></topicmeta></topicref>
<topicref href="conceptB.xml" type="concept" product="B">
<topicmeta><navtitle>Concept B</navtitle></topicmeta></topicref>
<topicref href="conceptC.xml" type="concept" product="A B">
<topicmeta><navtitle>Concept C</navtitle></topicmeta></topicref>
<topicref href="sub_filter.ditamap" format="ditamap"><topicmeta>
<navtitle>MAP TITLE</navtitle></topicmeta></topicref></map>

The second is referenced "sub_filter.ditamap"

<map xmlns:ditaarch="http://dita.oasis-open.org/architecture/2005/">
<title>Sub-DITA MAP</title>
<topicref href="conceptD.xml" type="concept" product="D">
<topicmeta><navtitle>Concept D</navtitle></topicmeta></topicref>
<topicref href="conceptE.xml" type="concept" product="E">
<topicmeta><navtitle>Concept E</navtitle></topicmeta></topicref></map>

I have a ditaval file as follows:

<val>
<prop att="product" val="A" action="exclude" />
<prop att="product" val="B" action="include" />
<prop att="product" val="D" action="exclude" />
<prop att="product" val="E" action="include" />
</val>

The problem I have is that the ditaval filtering is not working on the referenced ditamap, it only works on the top level.  In the above example: conceptA.xml was filtered out, but it failed to filter conceptD.xml.