Hmm you can't Compare Coercion with Agent Rex.
Coercion says as long as it is attached to the ally, it is under your control. As Coercion leaves play on the end of your turn the card it was attached to returns to his owner.
E.g you own all cards in your deck. When agent rex "steals" an Artifact you are still the respective owner, even if your oppenent controls it.
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