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Could be a Continuity Nod.
Or a Call-Back, or Once a Season. Grandfather Clause if it's so ingrained into the franchise that removing it would be blasphemous.
You've also got things like First Installment Wins, Adaptation Displacement, Sequel Displacement, and Audience-Coloring Adaptation for when one particular entry or adaptation in a series does something so popular that it shapes the common perception of the work- A classic example of several happening simultaneously is the shambling bolts-in-the-neck Frankenstein's Monster being the popular image rather than the disfigured but intelligent and athletic monster in the book, because that's the first version many people encounter.
Is there a trope for when a sequel does something that was in its predecessor just because it was in the previous film? I suppose it could apply to TV seasons too.