The poster is right, and a year later, the problema has not been dealt with. Yes, fine, is what the "clue" says. Dealing with Duolingo is too often a question of guessing what they will and will not accept, totally apart from logic.
In Duolingo you become clever to the extent thay you answer every thing correctly as long as app is open for you but once you close every thing goes out. It seems the app is intracting with your RAM- momory, not the the ROM.