Duolingo never tests you on new brand words in my experience. Whenever a new word is introduced, it is "clickable" (touchable?) in the sentence to show the definition. Maybe that's just my course though. Sometimes the new words are highlighed, other times (if I spend 20 gems for a "harder" lesson) they are not highlighted but still clickable.

The only time words are not clickable is during the legenrday challenges. I guess this is where you are seeing them.