The prefixes used in the embedded Schematron expressions from the docbookxi.rng (http://www.docbook.org/xml/5.1CR3/rng/) schema should be declared by using the ns{http://purl.oclc.org/dsdl/schematron} directive.
Please see the section 5.4.7 ns element from the ISO Schematron specification:
http://www.schematron.com/iso/P8.html
Thanks. That'll be fixed in the next CR release.