setting svg attributes with xsl only works if intermediate fo is created

When trying to set svg path attributes via a stylesheet, the resulting shape is only displayed when the pdf is created from the intermediate fo file.
If I skip the fo part and create the pdf directly the shape is not displayed. Same goes for the internal (-awt) viewer.

In the stylesheet the following lines causing the problem are:

<fo:instream-foreign-object>
<svg xmlns="http://www.w3.org/2000/svg" width="1.5cm" height="1.5cm">
<polygon style="fill: #cccccc;">
<xsl:attribute name="points"><xsl:value-of select="points"/></xsl:attribute>
</polygon>
</svg>
</fo:instream-foreign-object>

In short (files are attached):

$ fop -xml in.xml -xsl test.xsl -pdf out.pdf

Does not create correct output (but no warning/errors are given)

$ fop -xml in.xml -xsl test.xsl -foout out.fo
$ fop out.fo -pdf out.pdf

does work.