There are multiple issues at play here and its taken a bit for them all to be addressed.
Advanced sessions and SteamCore have both fixed the “uselobbieswhenavailable” bool that came into the picture and resulted in the default sessions nodes no longer working… their nodes expand the default calls to the OSS interface … however they didn’t resolve the issues with the actual module the interface proxy calls route to meaning clients who have started a session or who start one by default (friends only) would return false after a timeout when trying to run “find sessions” no matter what. This is a bug in later engine versions OnlineSubsystemSteam plugin. Searching for a session while also being a listen server host in these later versions would result in the following log
[2023.12.13-01.45.00:098][828]LogOnline: Warning: OSS: Async task ‘FOnlineAsyncTaskSteamFindLobbiesForFindSessions bWasSuccessful: 0 NumResults: 4’ failed in 15.096867 seconds
I’ve corrected this here GitHub - uno1982/UE4.27_OnlineSubsystemSteamFix: Fix to issues with UE4.27.2 binary engine module
You will still need to pair it with advanced sessions or SteamCore for full blueprint node support. and use their documentation or example projects to get going 