Necro BUMP!
Using UE 5.3.2 and also using Steam for sessions, and it was working great until I started getting this a couple of days ago. It’s driving me absolutely bonkers. It would be really nice to know why this happens and/or have an actual solution. Using “Open” in the console as a workaround isn’t really a work around at all. 
SOLUTION: Finally, after 3 days of madness; Turns out that even though ServerTravel was using a true value for it’s bAbsolute parameter, it was still loading the lobby seamlessly. Duplicating my “LoginGameMode” into a “StartGameMode”, then assigning the StartGameMode to the GameStartMap’s World Settings, and then disabling “Use Seamless Travel” in StartGameMode has fixed my issue. Why this started happening in the first place I still have no idea, but I’ll take the win. 