My document starts with

<?xml version="1.0" encoding="UTF-8"?>
<?xml-model href="http://docbook.org/xml/5.1/rng/docbook.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"?>
<!DOCTYPE chapter[
    <!ENTITY % my-entities SYSTEM "../Myentities.ent">
    %my-entities;
    ]>
<chapter version="5.1" xml:id="Ch11.Circuits" xml:lang="de" xmlns="http://docbook.org/ns/docbook"
    xmlns:xi="http://www.w3.org/2001/XInclude" xmlns:xlink="http://www.w3.org/1999/xlink">
    <title>Regulationsmuster</title>
    <para> In diesem Abschnitt ... </para>
    <sect1 xml:id="Ch11.S.FeedbackInhibition" xreflabel="ch11.S Feedback Inhibition">
        <title>Rückkopplungen</title>
 (snip)

however, the section numbering is shown as 0.1

AFAIC in Version 19 this was correct, but is a regression in 19.1

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