Eventually I figured this out by reviewing line-by-line my implementation against the AdvancedSession's. The fix was to add a few more parameters to our FindSession query to get it to properly pick up our Steam Sessions, which seems to require the Session be classified as a Lobby:
https://github.com/mrpropellers/megajam-2023/blob/main/Source/AntiquatedFuture/S...
SessionSearch->QuerySettings.Set(SEARCH_LOBBIES, true, EOnlineComparisonOp::Equals);
SessionSearch->QuerySettings.Set(SEARCH_PRESENCE, true, EOnlineComparisonOp::Equals);