There is this line in QbsProductNode::addFiles() (the same applies to QbsGroupNode):

    if (!prjNode || !prjNode->qbsProject().isValid()) {
        // error handling

For projects like qbs or Qt Creator, the second condition always seems to be true, so no file can ever be added. For some reason, the node cannot find a valid qbs project, even though there must be one, since the project tree is displayed correctly and building works as well.