Okay, slight rant to follow...Why in the name of heaven bother to put a single hint if it is incorrect. It sets the student up for failure. It is simply not fair. If you can't give the correct translation somewhere in the hints, then don't freaking bother to put the hint in at all!!! It makes me so angry...End of rant.
That does seem strange. I have never seen that happen before. Though Duo isn't perfect, usually when I believe that my perfect answer was marked wrong I eventually find some little error that I've made. Be sure to use the Report button so that eventually the moderators can review it.