"Professor, eins wollte ich von Ihnen noch hören."
well, this sentence makes no sense. In the moment it is spoken out, it is present. So it should be "I still want". i did this sentence wrong several (3) times, as the combination of "still" and "wanted" breaks my brain. "I wanted to hear one thing from you, but now I still don't want it anymore". Zum Glück mussten wir es nicht noch einmal machen. ..
The best I can offer to help your brain, is that "still wanted" is not present in the moment it is spoken out....so to speak. Try this..."I kept pestering my girlfriend because I still wanted to go to the bar, despite her telling me that she was too tired." That's the only way I can see "still wanted" working. As for Duolingo's sentence, it is incomplete. If they added to it along the lines of my example sentence it might work. As it stands though, terrible sentence.
To be fair, such a formulation is not unusual in English (I couldn't say if this is the case in German). It is a polite form of address, such as can be formulated both in the past tense as well as in the conditional (or a combination of the two) in English where a sort of self-effacing play of temporality and modesty puts the fate of your desire/wish in the hands of the person addressed. A banal example would be -- Upon entering a hairdressers and confronting the person behind the counter who asks how they may help you, you might say: "I wanted to know whether I could get my hair cut this afternoon?" / "I was hoping to get my hair cut this afternoon." In both cases the past tense can be thought of as operating in[to] the present.