Marked as wrong for "Anna writes more slowly than Tom" (which is correct), and given solution of "Anna writes slower than Tom" (which is incorrect).
This is because "slower" is an adjective (ie, used to describe nouns), whereas "more slowly" is an adverbial construction and is what is needed here (as we're describing the action of writing, not Anna per se).
"slow" is down as an adverb in the dictionary so "slower" is also adverbial.
Actually, we have not added that much since the launch of the course in early November 2015. It is just that Duo automatically gathers statistics and mostly gives you sentences users have least trouble with. For example, Это быстрая песня is offered about three times as often as Мы знаем много песен.
The system makes your life easier but ultimately may leave the impression that some lessons lesson have, like, 5 or 6 sentences while in reality they have dozens.