The order that OnAfterDeserialization will be called on nested objects is not guaranteed, so an object should not assume that OnAfterDeserialization has already been called on a different object when doing its own initialization. A recent fix guarantees that SerializeReference fields will be fully initialized by the time OnAfterDeserialization is called. And it is also guaranteed that OnAfterDeserialization on the outer object would be called last. It is our recommended to trigger any initialization code that needs to span across nested objects to be triggered from the main object's OnAfterDeserialization callback, that is a safe way to avoid fragile dependencies on the order of callback execution.