Check the attached example. Click the window couple times and app crashes. This is a simple example, bug appears also with Quick3D particles testbed.

So when Loader Item contains View3D which has some animation, unloading Loader causes a crash.

Debug output:
1 std::_Atomic_storage<int,4>::load atomic 604 0x7fffe92976ff
2 QAtomicOps<int>::loadRelaxed<int> qatomic_cxx11.h 238 0x7fffe929bcfa
3 QBasicAtomicInteger<int>::loadRelaxed qbasicatomic.h 90 0x7fffe92ba3eb
4 QArrayData::needsDetach qarraydata.h 107 0x7fffe92f7e7b
5 QArrayDataPointer<QSSGRenderGraphObject *>::needsDetach qarraydatapointer.h 222 0x7fffe93ccf26
6 QArrayDataPointer<QSSGRenderGraphObject *>::detach qarraydatapointer.h 165 0x7fffe93cac1d
7 QList<QSSGRenderGraphObject *>::detach qlist.h 351 0x7fffe93cadd4
8 QList<QSSGRenderGraphObject *>::begin qlist.h 537 0x7fffe93c96fd
9 QSSGRenderer::cleanupResources qssgrenderer.cpp 124 0x7fffe93b4b97
10 QSSGRenderContextInterface::cleanupResources qssgrendercontextcore.cpp 161 0x7fffe92d4798
11 QQuick3DSceneRenderer::cleanupResources qquick3dscenerenderer.cpp 577 0x7ffffba58dde
12 <lambda_8e06d9bb3b968ccdd6f000303f89bbed>::operator() qquick3dscenerenderer.cpp 256 0x7ffffba6311b
13 QtPrivate::FunctorCall<QtPrivate::IndexesList<>,QtPrivate::List<>,void,<lambda_8e06d9bb3b968ccdd6f000303f89bbed>>::call qobjectdefs_impl.h 146 0x7ffffba644cd
14 QtPrivate::Functor<<lambda_8e06d9bb3b968ccdd6f000303f89bbed>,0>::call<QtPrivate::List<>,void> qobjectdefs_impl.h 257 0x7ffffba5f7c7
15 QtPrivate::QFunctorSlotObject<<lambda_8e06d9bb3b968ccdd6f000303f89bbed>,0,QtPrivate::List<>,void>::impl qobjectdefs_impl.h 426 0x7ffffba65051
16 QtPrivate::QSlotObjectBase::call qobjectdefs_impl.h 375 0x7fffea97a4f7
17 doActivate<0> qobject.cpp 3825 0x7fffeaf8fb97
18 QMetaObject::activate qobject.cpp 3886 0x7fffeaf78fac
19 QQuickWindow::afterFrameEnd moc_qquickwindow.cpp 659 0x7fffe994a7a9
20 QSGRenderThread::syncAndRender qsgthreadedrenderloop.cpp 832 0x7fffe9cea3c7
... <More>