We encountered the same issue. Seems the STEAMKEY_NUMOPENPUBLICCONNECTIONS key is never updated.
In Session.NumOpenPublicConnections = FCString::Atoi(ANSI_TO_TCHAR(Value)); just forever gets the default value.
Only after a player joined, the player calculates it again, this time with:
Session.NumOpenPublicConnections = MaxLobbyMembers - LobbyMemberCount;
There is code (FOnlineAsyncTaskSteamUpdateLobby) that looks like it should update NumOpenPublicConnections, but it’s never called:
UE_LOG_ONLINE(Verbose, TEXT("Updating Lobby Data (%s, %s)"), *It.Key(), *It.Value());
if (!SteamMatchmakingPtr->SetLobbyData(SessionInfo->SessionId, TCHAR_TO_UTF8(*It.Key()), TCHAR_TO_UTF8(*It.Value())))
If you decide to call it, it just destroyed the entire session and nobody can ever join it again.
Epic decided to delete all steam keys, but instead of deleting them with DeleteLobbyData, they just change it to ‘’
UE_LOG_ONLINE(Verbose, TEXT("Removing Lobby Data (%s, %s)"), *It.Key(), *It.Value());
if (!SteamMatchmakingPtr->SetLobbyData(SessionInfo->SessionId, TCHAR_TO_UTF8(*It.Key()), ""))