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I had a Etherium-Horn Sorceror out, and the AI summons Aether Adept and then bounces itself back to hand, and does nothing else for that turn. Is the AI more prone to making bad plays if it uses a custom deck?

Custom decks will not change the behaviour of the AI. Although, saying that, giving the AI a deck which cannot win, or is more likely to kill yourself, will have an impact.

Without knowing more about what was on the battlefield, or in the AI's hand, it looks as though it more to do with the bounce target-picker.

Choosing the Etherium-Horn Sorcerer wouldn't be a good move as it has Cascade, and would give you another spell/permanent. Choosing a permanent the AI owns with an enter the battlefield ability would normally be a good move, but not if it's the permanent that triggered the ability. We wouldn't want to hard-code Aether Adept to only target another permanent, as if there was another enter the battlefield trigger/when spell is cast on another permanent it would make sense to bounce itself. (Æther Flash Animar, Soul of Elements)

It could also be that through all the game simulations the AI has done, it doesn't make a difference if it has the Æther Adept in play or not, and that the mana couldn't be used for anything else that turn.

It's still a poor play though - The AI type you have chosen and the level (thinking time) will generally change the behaviour of the AI, and will also change depending on the specs of your pc.

Thank you very much!