Champion Select Bug

Hey guys, I recently noticed a bug where in the champion select chat it says one of the people that the system match me with left the room and the champ select kept going everything else was going normal but when the actual game start that one teammate who left the room become immediately afk, so I lost a 4v5 ranked because of that. I also saw that today in a ranked champ select so i told my friend to quit because he still had enough lp to make it into promo within 1 game after he dodge. Can anyone tell me if that player just simply exit the client during the loading screen or its a riot problem. Thank. (I wasn't playing on this account, the IGN for that acc is Shadow HYS)