Looks like this line in LobbiesCommonTypes.cpp::668 is a copy/paste error
for (TSharedRef<FLobbyMemberInternal>& LeavingRemoteMember : LeavingRemoteMembers)
{
RemoveMember(LeavingMember);
Params.LobbyEvents->OnLobbyMemberLeft.Broadcast(FLobbyMemberLeft{ InternalPublicData, LeavingRemoteMember, ELobbyMemberLeaveReason::Left });
}
Should actually be RemoveMember(LeavingRemoteMember)
Otherwise, when the callback for LobbyLeft triggers, there are still members in the lobby that has been left