I submitted a support request about this, and they said the devs are aware of the problem and a working on a proper fix. In the meantime, however, the advice was:
"Try performing a soft reset, which is where you log out of the game and launcher completely for at least 15 minutes. This will reset your character's instance information and may cause quest NPCs to work properly.
You can also try to abandon and reacquire this quest, which also causes quest NPCs and objectives to restart with positive results."
I tried this, and it worked!
https://www.youtube.com/watch?v=M4Wartv7MQ8