Philip Newton's journal

This is what I was thinking - in earlier seasons, their relationship was more formal, more student-teacher. My knowledge of German linguistics is pretty much non-existant, but maybe they thought using "Du" could make people think there was some sort of non-professional interest in each other? Maybe also because of the age difference and that she should use "Sie" as a sign of respect and he would use it because he doesn't want to get too close to her (or at least wants it to sound like he's not getting too close to her)?