Links with anchors are not converted during import. Example:

<a href="targetPage.html#myAnchor">Jump</a>

should be converted to something like

<a href="https://example.com/mod/book/view.php?id=123&chapterid=456#myAnchor">Jump</a>

But the link remains untouched. The following change takes this into account.

Affected file: mod/book/tool/importhtml/locallib.php

Function: toolbook_importhtml_import_chapters(...)

Current source code:

$chapterpath = dirname($chapter->importsrc).'/'.$matches[2][$i];
$chapterpath = toolbook_importhtml_fix_path($chapterpath);
foreach ($allchapters as $target) {
    if ($target->importsrc === $chapterpath) {
        $newcontent = str_replace($match, 'href="'.new moodle_url('/mod/book/view.php',
                array('id'=>$context->instanceid, 'chapterid'=>$target->id)).'"', $newcontent);
           }
}

Proposed change:

$anchor = explode("#", $matches[2][$i]);
$chapterpath = dirname($chapter->importsrc).'/'. $anchor[0];
$chapterpath = toolbook_importhtml_fix_path($chapterpath);
foreach ($allchapters as $target) {
    if ($target->importsrc === $chapterpath) {
        $newcontent = str_replace($match, 'href="'.new moodle_url('/mod/book/view.php',
                array('id'=>$context->instanceid, 'chapterid'=>$target->id), $anchor[1] ?? null).'"', $newcontent);
           }
}