How to detect landstreamingproxy was added in world composition

Hello,

I am using world composition to represent a large area for simulation purpose.
I wanna get a callback when a new landscapestreamingproxy was loaded into the level.
I was using delegate to try catch the new added landscapestreamingproxy but somehow no landscapestreamingproxy
was ever called into the delegate:
FOnActorSpawned::FDelegate::CreateUObject(this, &ULandscapeTaggerDelegate::OnActorSpawned)
void ULandscapeTaggerDelegate::OnActorSpawned(AActor* actor)
{
if (actor != nullptr && actor->IsA(ALandscapeStreamingProxy::StaticClass())) {
ALandscapeStreamingProxy* proxy = Cast(actor);
if (proxy) {
proxy->bRenderCustomDepth = true;
proxy->CustomDepthStencilValue = (int)EObjectSemanticLabel::Roads;
}
}
}

Can anyone point me out anything wrong or any other way to do it? Much appreciated.