I think these three are all connected, but I would like to add my experience since it might have useful details.
Recently the system started sending these joining/leaving messages. I’m actually fine with that because I can see who I’m joining and who the host is, although I don’t think it was desired behaviour.
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During at least a couple of games there would be a message about one or more players leaving. They would usually stay in the game, although on at least one occasion the player leaving would actually appear as a bot for up to 10 seconds. The players didn’t leave because there is no way for anyone to rejoin the game so quickly. I cannot say if on the occasion when HUD showed “bot” in the character name the player was actually replaced by the bot.
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In one game - and this may be the other side of “full lobby join bug”; I was the host on that occasion - I saw the message about a player leaving and joining, although the player stayed in the game all the time. However, from that point onwards, there were at least 5 failed attempts to join the game by other players.