I've found I get different results with xsl:for-each. Would you try the following which uses xsl:for-each instead of xsl:if?

<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Tr
+ansform">
<xsl:template match="/">
<xsl:for-each select="/PLMXML/Form[@id=id24]">
    <xsl:value-of select="@id"/>
</xsl:for-each>
</xsl:template>
</xsl:stylesheet>
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This xslt returns id36 for me.