vine: tasks stuck in READY → RUNNING → WAITING_RETRIEVAL → READY loop

I've occasionally run into the case where most of the tasks were scheduled and retrieved normally, while the last reduction task gets stuck in the waiting list.

<img width="1578" alt="Image" src="https://github.com/user-attachments/assets/72ff6fe4-3e5c-4e5f-aa59-f6da9aeefab0" />