Phabricator was upgraded on July 13th and that upgrade included the new "Task Graph" functionality. This graph functionality is unable to deal with very very large sets of dependent tasks (eg: many of our tracking tasks or other tasks with many sub-tasks or tasks being subtasks of tracking tasks). This initially prevented the task page from loading at all.
We have committed a temporary fix (authored by upstream) to disable the graph on tasks that have more than 100 related (sub- or parent) tasks.
Temporary workaround for lost functionality:
For tasks that the list of sub-tasks and parents tasks are not shown you can view them via "Edit related tasks" and selecting either "Edit Parent Tasks" or "Edit Subtasks" as appropriate. The full list will be displayed there and you can click on the icon to the left of the task title to open the task in a new browser tab.
A real fix is in the works upstream to deal with this corner case more gracefully.