I’ve figured out what was happening; maybe this will help someone else as well. I’ve been running a dedicated server for the whole project and as a result I’m always hitting Play with Net Mode set to ‘Play As Client’. Of course this meant the Bootstrap map also spawns a dedicated server which would then run the same BeginPlay events, thus the server would swap levels with the client still connected (and probably also traveling at the same time), crashing the engine.
This was easily solved by a simple ‘Is Server’ or ‘Is Dedicated Server’ branch on False. (Or just run Net Mode as ‘Play Offline’ in the bootstrap map).