The heading bookmarks in PDF metadata are placed at wrong levels. In the PDF document, the chapter headings and table of contents are rendered as expected.
The misplaced bookmark levels are a bug within PDF postprocessing via ghostscript. As a workaround it is possible to disable PDF compression by settings COMPRESS_PDFS=off in app.env.
The heading bookmarks in PDF metadata are placed at wrong levels. In the PDF document, the chapter headings and table of contents are rendered as expected.