Apparently, as you say, the reveal that it was definitely Joffrey who hired the assassin doesn't come until Tyrion is escaping and talking with Jaime in the dungeon. But now if they have that reveal happen, it'll just seem like a random "oh, by the way, your dead son was SUCH a douchebag he even hired an assassin to kill a cripple."

I suspect they'll just ignore it, because at this point Tyrion is the only one alive to whom that fact kinda matters and most viewers probably forgot that there was even a mystery about who hired the assassin.