Hopper Graphic Designer interview questions

The interview process started great and got to meet with a few people on the team for the first 2-3 interviews. Then, they tasked me with a design exercise to come up with an ad concept they could run. I put in a lot of time and effort into this and after submitting, got zero response/feedback from the actual design team and instead a generic rejection from their HR dept. This felt like a waste of time after thinking I was making real progress in the process, but maybe they're just trying to farm new ideas (the posting was still live months after interviewing).