This web article says:
Astatic needles are basically two needles of approximately the same magnetic strength, mounted on top of one another. They are parallel, but their magnetic poles are on opposite sides, as shown in the diagram. This way, when inside (any) external magnetic field, the torque exerted on one needle is equal and opposite to the torque exerted on the other. Since they are connected, the system overall remains unaffected by the field.
Thus it seems to me that astatic needles should also be immune to other magnetic fields. Why is it not so?