I've found a reproducable error when nesting a set in another set (DocBook 5.1 using Oxygen 18.0). I thought that it might be caused because the set contained articles (new for DocBook 5.1) but it appears again when the set contains books. I have reproduced both below.
This affects chunked HTML and EPUB transforms. Both examples are a set (set A) containing one article/book and another set (set B) containing one article/book.
After transform, the articles and books appear in the 'Previous'/'Next' links and EPUB transforms, but the abstract of Set B does not. You can reach the Set B abstract by the table of contents in the Set A (first chunk in HTML output) but not by moving through the document.
The two examples are very similar. First example (articles)
<?xml version="1.0" encoding="UTF-8"?>
<?xml-model href="" href="http://docbook.org/xml/5.1/rng/docbookxi.rng">http://docbook.org/xml/5.1/rng/docbookxi.rng" schematypens="
http://relaxng.org/ns/structure/1.0"?>
<?xml-model href="" href="http://docbook.org/xml/5.1/sch/docbook.sch">http://docbook.org/xml/5.1/sch/docbook.sch" type="application/xml" schematypens="
http://purl.oclc.org/dsdl/schematron"?>
<set xmlns="
http://docbook.org/ns/docbook" xmlns:xi="
http://www.w3.org/2001/XInclude"
xmlns:xlink="
http://www.w3.org/1999/xlink" version="5.1">
<info>
<title>Set A</title>
<abstract>
<title>First set</title>
<para>This is a set of two articles, one of which is in another set</para>
</abstract>
</info>
<article>
<title>Here is an article title</title>
<info>
<abstract>
<title>Title of an article abstract</title>
<para>Text of abstract</para>
</abstract>
</info>
<simplesect>
<title>Title of simple section</title>
<para>Text of simple section</para>
</simplesect>
</article>
<set>
<info>
<title>Set B containing one article in a set</title>
<abstract>
<title>Second set</title>
<para>Is this text visible?</para>
</abstract>
</info>
<article>
<title>Here is an article title</title>
<info>
<abstract>
<title>Title of an article abstract</title>
<para>This article is in another set. Did you see the abstract of set B?</para>
</abstract>
</info>
<simplesect>
<title>Title of simple section</title>
<para>Text of simple section</para>
</simplesect>
</article>
</set>
</set>
Second example (books):
<?xml version="1.0" encoding="UTF-8"?>
<?xml-model href="" href="http://docbook.org/xml/5.1/rng/docbookxi.rng">http://docbook.org/xml/5.1/rng/docbookxi.rng" schematypens="
http://relaxng.org/ns/structure/1.0"?>
<?xml-model href="" href="http://docbook.org/xml/5.1/sch/docbook.sch">http://docbook.org/xml/5.1/sch/docbook.sch" type="application/xml" schematypens="
http://purl.oclc.org/dsdl/schematron"?>
<set xmlns="
http://docbook.org/ns/docbook" xmlns:xi="
http://www.w3.org/2001/XInclude"
xmlns:xlink="
http://www.w3.org/1999/xlink" version="5.1">
<info>
<title>Set A</title>
<abstract>
<title>First set</title>
<para>This is a set of two books, one of which is in another set</para>
</abstract>
</info>
<book>
<title>Here is a book title</title>
<info>
<abstract>
<title>Title of an book abstract</title>
<para>Text of abstract</para>
</abstract>
</info>
<chapter>
<title>Chapter title</title>
<simplesect>
<title>Title of simple section</title>
<para>Text of simple section</para>
</simplesect>
</chapter>
</book>
<set>
<info>
<title>Set B containing one book in a set</title>
<abstract>
<title>Second set</title>
<para>Is this text visible?</para>
</abstract>
</info>
<book>
<title>Here is a book title</title>
<info>
<abstract>
<title>Title of an book abstract</title>
<para>Text of abstract. Did you see the abstrract of set B?</para>
</abstract>
</info>
<chapter>
<title>Chapter title</title>
<simplesect>
<title>Title of simple section</title>
<para>Text of simple section</para>
</simplesect>
</chapter>
</book>
</set>
</set>