As reported in #3686, the locally loaded rating of the current lobby user is not always in sync with the rating seen by other players.
This can occur if a player (for an unknown reason) didn't receive the rating list (yet?) from the lobby bot when joining the lobby.
It can also occur when having played a rated game, returned to the lobby and then having the rating adapted.
There might be more cases.
Hence the rejoin won't work for these users anymore and the player might rejoin as a late observer or not being able to rejoin at all if late observers are disallowed.
Both cases are very bad as they can end the game unintentionally (and players might not believe each other that this was a bug).
Not saving the rating in the nickname is an idea, but it will come with complications (changing every place that displays the playername in the gamesetup, session, summary and replaymenu GUI).