This sounds like the exact problem I am having in 4.17.2
I have seamless travel set in transition and my main map, i use subsystem = null for just a dedicated server on my LAN.
As you mentioned the client finds the first player start and just sits there unresponsive to anything. Do you have any thoughts on how to fix or resolve this through blueprints? How would I wait to the server is loaded before bring clients to the level or if they do arrive early, have them reload the map?