Duolingo's tolerance of mistakes is weird.
One question let me get away with 'sozial' when I should have typed 'soziale'. Another question in the same exercise marked 'deutsche' wrong when it should have been 'deutschen'.
Was ist mit Duolingo?
When I complained about mistakes in another language I was told that every sentence is coded individually with a set of acceptable answers. If two developers enter similar sentences, then it's possible that the same variant will be accepted in one but not the other.
Over time these inconsistencies get noticed, reported, and corrected. But it's not a perfect process and even long-established languages (like German) have inconsistencies and flat-out mistakes, especially towards the bottom of the tree. But look at the bright side: in newer DL courses it's a lot worse!