SPOILER WARNING
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Idk if you already solved this, but maybe try being more eager/exited about the whole in-game ordeal. What I did was focusing on Tenebris without being rude to Keith, just more of indifferent. Then choose the options where MC is visibly thrilled about the danger. You may also wanna try changing your plant option at the start of the game, I'm not 100% sure if this has to do with it but I did change it after a few tries and one of these combinations eventually worked, so..