According to their support: "We’ve uncovered a technical issue on web and we are disabling HD playback for movies until we are able to fix the issue. As of now

According to their support: "We’ve uncovered a technical issue on web and we are disabling HD playback for movies until we are able to fix the issue. As of now, we don't have any timeline on when this issue will be fixed. However, users can still stream their movies or TV shows in HD on other supported devices."