Duolingo has the same issue most similar language learning apps do: as you progress the lessons degrade in quality compared to a textbook. This means at a more advanced level you'll get inconsistent answers, and often very poor English appears after Unit 2 or earlier. I think it's not ethical to sell subscriptions for an app that drops the ball this hard. It's so heartbreaking to learn the app didn't even teach me posessives correctly in Vietnamese. What is the point?