Dynamically Spawned Level Instance Actors do not have their sub actors world partition stream in PIE starting level

We are building levels where we spawn in Level Instance actors at runtime that have World Partition enabled into a Persistent level that also has World Partition enabled. What we have noticed though that none of the actors that are a part of the level instance stream in and out when playing in PIE for the level you start in. They properly stream if we transition to a different level and back while playing. It also works correctly in Stand Alone.

After some digging, it appears it is related to the call ILevelInstanceInterface::SupportsPartialEditorLoading() returning false if the Persistent level has the PKG_NewlyCreated flag.

ILevelInstanceInterface::SupportsPartialEditorLoading()

	// If the level instance actor (or any parent actor) is unsaved, don't partially load.  
  	if (Actor->GetPackage()->HasAllPackagesFlags(PKG_NewlyCreated))  
  	{  
  		return false;  
  	}  

However, by default the engine will always add PKG_NewlyCreated to the PlayWorldPackage for the Persistent Level you are starting PIE in

UEditorEngine::CreatePIEWorldByDuplication

UPackage* PlayWorldPackage = CreatePackage(*PlayWorldMapName);  
// Add PKG_NewlyCreated flag to this package so we don't try to resolve its linker as it is unsaved duplicated world package  
PlayWorldPackage->SetPackageFlags(PKG_PlayInEditor | PKG_NewlyCreated);  

Unsure if this is the appropriate fix as there are other places in the Level Instance / Streaming code that check against PGK_NewlyCreated for other reasons, but I was able to get the runtime spawned Level Instance streaming to work in the level you start PIE in with the following change:

ILevelInstanceInterface::SupportsPartialEditorLoading()

// If the level instance actor (or any parent actor) is unsaved, don't partially load.  
if (Actor->GetPackage()->HasAllPackagesFlags(PKG_NewlyCreated))  
{  
	if(!Actor->GetPackage()->HasAllPackagesFlags(PKG_PlayInEditor)) // CHANGE - Exclude PlayInEditor types of NewlyCreated  
		return false;  
}  

In PIE, spawn a Level Instance Actor at runtime and set its world asset.