To reproduce:

1. Create a task with both view/edit polity set as "Subscribers".
2. Set the Subscriber of the Task to a parent project.

Result: No one in the parent project can view/edit the task unless he is explicitly specified as a subscriber or the owner/creator of the task.

If the project is a normal project without subprojects, above steps produce expected result. Everyone in the project can view/edit the task. I believe it is caused by that parent project actually has no direct project members.