I have recently come across this question myself whilst exploring the data streaming capabilities of UE4, I’m posting this in case somebody else comes across this thread one day.
The following presentation by Alan Willard is what you’re looking for: Getting Started with Level Streaming | Live Training | Unreal Engine - YouTube
For the specific scenario that you are describing, he provides the following two recommendations:
1 - Rather than unloading the level where your AI agents are working, turn off its visibility, as it seems that logically your game design still wants that level existant in memory (for the case that your AI agents are interacting with their environment whilst the player is somewhere else in the game).
2 - You could maintain a persistent collision base for the world, so that even if the streamed level the agents are traversing has been unloaded, you have the consistency that they won’t fall off the world, however, this does mean that their interactions are limited to movement only, as unloading the level they are contained in will destroy the objects they could interact with.
What I would personally do is simulate ai interactions in the persistent level, then, when the streamed level containing the ai is constructed, use the properties produced by the simulation to initialize the AI.