Grandchild actor with IsEditorOnly enabled is duplicated during save the level.

void UChildActorComponent::CreateChildActor()
{
    // ...
#if 1 // changed start
    bool isEditorOnly = IsEditorOnly();
    if (MyOwner)
    {
        isEditorOnly = isEditorOnly || (MyOwner->IsEditorOnly() || MyOwner->HasAllFlags(RF_Transient));
    }
    if(HasAllFlags(RF_Transient) || isEditorOnly)
#else // original
    if (HasAllFlags(RF_Transient) || IsEditorOnly())
#endif // changed end
    {
        // If we are either transient or editor only, set our created actor to transient. We can't programatically set editor only on an actor so this is the best option
        Params.ObjectFlags |= RF_Transient;
    }
    // ...
}