So after all of that, it turns out that despite our clients all using non-seamless/absolute travel to go to the host, the host themselves was not using non-seamless travel.
Once MrMightyErik commented above, it got me thinking (and then realising) that the host wasn’t using non-seamless but that the console command would have triggered a non-seamless travel to the restart/re-host.
Quick tip for anyone else who ends up here: Make sure your host is initialising their session and host game using non-seamless travel. Once into your multiplayer game mode, they should then be able to use seamless travel to continue the game loop (end match > new match etc).