The new QDoc has a new scope mechanism for finding links. In qtdoc, many of the links point to the index landing pages, but since there are section titles with the same name as the landing pages, QDoc resolves them to the one found in qtdoc repository.

See: http://doc-snapshot.qt-project.org/qt5-5.4/qtmodules.html The links to Qt Core and Qt GUI go to the section titles.