This was reported a while back and was logged as JIRA UE-40515. It was determined to be By Design by the developer. Spawned actors will always spawn to the persistent level.
This behaviour is causing me trouble too.
I am using Level Streaming to pre-load maps instead of extending a map/environment with new sections and in my case the current behaviour is un-helpful.
It would be great if there was a separate way to pre-load maps, to speed up location/level switches without using Level Streaming since it does not seem to be designed for that.