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In practice, I can say that even when there is no message indicating a disconnection and the player has left the game, the following may occur.
I had a situation like this (at least twice) where I didn't receive any message that the player had left the game, and neither did the player. Everything seemed normal. I attacked the opponent's army, defeated it, and then the opponent told me that I had attacked him, but didn't kill him, and instead allowed him to buy his way out. The disconnection occurred after the battle, although neither I nor the opponent received any message about a disconnection before or during the battle.
I had no reason to doubt my opponent, as he claimed that he saw one thing, and I saw something else.
But this is just an aside, and the AK will review the situation regarding this "suspicious" game and make a decision.