An OpenGL-specific issue about missing or incorrectly rendered objects in the scene has now been confirmed to be happening in non-core profile contexts only (on desktops, such as Windows or Linux with NVIDIA).
It is quite visible when running scenes with qmlscene, e.g. Lancelot test cases.
qmlscene --rhi gl something.qml will lead to missing objects, and different results on almost every run.
qmlscene --core-profile --rhi gl something.qml is always correct.
The Lancelot test machines, examples, and any application that calls QSurfaceFormat::setDefaultFormat(QQuick3D::idealSurfaceFormat()); in main() will not be reproducing this because we prefer core profile contexts on the desktop.