Same as many people, I passed multiple rounds of their (extremely long) process, only to end up being ghosted after spending 6 hours on a work task. The role was advertise for months if not years after that happened, which makes me really wonder what was going on.

Two interviews and then a challenging 4-hour task during which you had to identify and explore an issue for high-impact change. The time was extremely limited for such a task but it was acknowledged from the very beginnning that the task was intentionally very challenging.