1. Have a multi-module-layout in Maven
2. Configure doclava as specified for Maven in parent POM
3. Run mvn clean site
4. See the site's javadoc being generated to ${project.build.directory}/site/apidocs
5. The file ${project.build.directory}/site/apidocs/index.html is an empty file.
6. The "real" doclava Javadoc starts in ${project.build.directory}/site/apidocs/docs/index.html.
7. The maven-site-plugin will link to the empty index.html

I assume that it's just a problem of the additional "docs" folder.

I also see this problem if doclava is specified in the child module

If you specify a destDir, then it becomes ${destDir}/docs IE: if you try to set ${project.build.directory}/site/apidocs/docs to circumvent this bug, you instead get ${project.build.directory}/site/apidocs/docs/docs