So, I play a match today on London Underground. And, as you probably guessed, half the players disconnected before the train arrived at the station. Usually this isn't a problem, as the servers usually automatically get some random players to fill the slots, but that never happened and the entire game was just a 3 v 2 match that just dragged on.
Then it's another 5 v 5 deathmatch, by the end of the game, it was 5 v 2 because, again, people leave, no one replaces them.
Now here's the fun part. On the trains on London Underground, within 3 minutes, all five players from the other team had left with no replacements and the game ended. That's got to be a new record or something.
TLDR: The server is for some reason doing a terrible job putting needed players in games in progress.