There has been a recent (?) bug of players attempting to connect to a custom map, appearing to be in the lobby for 1-2 seconds, and then disconnecting. They repeatedly try to join but are automatically disconnected. I’ve experienced the same when attempting to join a lobby. All of my friends have seen it and experienced it as well.
In my opinion, this is the single biggest problem in the game, as it appears to affect half of players who have not previously played a particular map, effectively halving the join rate of my hosted lobbies. I hope it’s a priority fix.