I love the app, and it helped it me a lot rabbi ybarek fikom.
I am using and practicing daily but it seems I keep getting the same vocabulary of Quranic words when I do the section “practice your vocabulary”.
I also use the app to read the Quran itself and mark words that I do not know the meaning of as “not familiar” but it seems they do not pop up when I do the test to learn new words.
My understanding is that the “daily challenge” is with verses containing words that you do know. Words marked as unkown are counted as potential new words for you to learn (“study”), but any verse containing unknown words won’t show in the challenge.
As for when new which words are learned, I think they appear according to frequency in the Qur’an and if you prioritize a surah, they appear in order of frequency among the prioritized surah(s).