Co-op battle ended immediately. Lost 20 stamina.

So I joined a co-op battle but after the first few seconds, I received a pop-up that the battle ended immediately. No rewards, lost 20 stamina. Don't know if the host quit or a connection issue, but the thing is the other player shouldn't be penalized from it. Maybe have the game control the player who quit so the other player can still play and get rewards.