I struggle with pagebreak before sections level 1.

I have this start of the file:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-model href="http://docbook.org/xml/5.1/rng/docbookxi.rng"
schematypens="http://relaxng.org/ns/structure/1.0"?>
<?xml-model href="http://docbook.org/xml/5.1/sch/docbook.sch"
type="application/xml" schematypens="http://purl.oclc.org/dsdl/schematron"?>
<?xml-stylesheet type="text/xsl" href="../xslt/fo/Falterbuechlein.xsl"?>
<book xmlns="http://docbook.org/ns/docbook" xml:lang="de"
xmlns:xi="http://www.w3.org/2001/XInclude"

Falterbuechlein.fo looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:xd="http://www.oxygenxml.com/ns/doc/xsl"
    exclude-result-prefixes="xs xd"
    version="2.0">
    <xd:doc scope="stylesheet">
        <xd:desc>
            <xd:p><xd:b>Created on:</xd:b> Aug 4, 2018</xd:p>
            <xd:p><xd:b>Author:</xd:b> bk</xd:p>
            <xd:p></xd:p>
        </xd:desc>
    </xd:doc>
    <xsl:attribute-set name="section.level1.properties">
        <xsl:attribute name="break-before">page</xsl:attribute>
    </xsl:attribute-set>
</xsl:stylesheet>

However, it is not used. What to do to make it working?

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