When level instance is in standalone mode, the actor with IsMainWorldOnly enabled is loaded into persistent level.

The IsMainWorldOnly setting for Actor in LevelInstance is not reflected when LevelInstance is in Standalone mode. The setting is respected when LevelInstance is in Embedded mode.

The Actor is not loaded when the LevelInstance is placed in Standalone mode, but is loaded when the session is started in PIE. This can be a issue if working with directional lights in sublevels and do not want to use them when integrating into the main world. 

1. Create a open world map as a level instance.
2. Place a directional light actor and set IsMainWorldOnly=true.
3. Create a open world map as a persistent level.
4. Place a directional light actor.
5. Place level instance created by step1 into persistent level.
6. Set LevelBehavior=Standalone in level instance.
7. Play in editor.

Result:

Directional Light Actor in SubLevel is not loaded

Expected:

Directional Light Actor in SubLevel is loaded

Just PIE the attached project to see this.