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A "surprise discount" banner whose magic wears off

A food-delivery app tests a playful "surprise discount" banner on the home screen with a random half of users and measures orders per user. Orders are up 6% in week 1, 4% in week 2, 2% in week 3, and back to flat by week 4.

What is the most likely explanation, and how would you confirm it before deciding to keep the banner?

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