I just retrieved the confirmation that this bug happens unrelated to being in a game or not. So just loosing connection to the lobby and rejoining it, might trigger it.

I just created a thread in the bug reports subforum dedicated for this problem: Let's continue the discussion about that bug there, so this thread can be used for discussing the mod instead.

There have been reports that rejoining games isn't possible sometimes, as the rating for the player rejoining the lobby doesn't show up (https://gitea.wildfiregames.com/0ad/0ad/issues/7185). There was already some discussion regarding this topic in the following thread: Analysis of logs for cases where that happened to players showed that in these cases the players show up as connected with two different instances of 0ad to the lobby server. As we can assume they didn't do that intentionally, it's still not clear how that can happen. Workarounds which would fix this problem as well would be https://gitea.wildfiregames.com/0ad/0ad/issues/5320, https://gitea.wildfiregames.com/0ad/0ad/issues/4203 or patching Slixmpp to let EcheLOn keep track of all connected resources of a player, however these would just be workarounds, without addressing the root cause. What I need for further debugging to find the root cause are: whenever this problem happens to you, ping me in the lobby whenever this problem happens to you, reply here with the following information: operating system kind of internet connection (landline, mobile network, satellite, ...) list of installed mods reason why you left the game in the first place (crash, loss of internet connectivity, ...)

It's been happening since pre-2017. There used to be a workaround where you would join via IP. It would also allow you to replace players if someone leaves mid game. But this workaround got closed in a22. https://trac.wildfiregames.com/ticket/3549 #3549 seems really unnecessary and causes more problems than it's worth. But I digress  It is also worth noting that while players sometimes get temporarily locked out of games because their rating disappear there are also other instances where games get permanently ruined because a player initially joins a lobby without any rating. Then the game believes they have no rating. But if they drop and try to return their rating appears (as it is supposed to). As a result, the game doesn't recognize them and they are unable to rejoin a game unless this (relatively rare) bug reoccurs. 

That’s interesting that the cases involved multiple instances. 

The connection thinks that it is alive even after I have killed all pyrogenesis processes in the system. When I restart 0ad and rejoin the lobby 30 seconds later, there is still no game list (previous session is considered active by the bot). Not sure why that is.

Disconnecting is pretty common when players have spotty internet or otherwise poor connection. It could also be crashes, or restarting to fix errors.