The Qt3DExtras::Qt3DWindow has an attribute m_root, another one called m_rootEntity and a method setRootEntity. During the expose event, m_root is set as the root entity of the RenderAspect and m_rootEntity, if not null, is set as a child of m_root.

After the expose event it's still possible to call setRootEntity, but the the new m_rootEntity is never set as child of m_root, so it has no effect in the scene.