how does a compass behave when it is at true south somewhere in antarctica?

Answer 1

It will point towards the magnetic south pole.
The two sites are not in the same place.
In fact, at the present, the Magnetic South Pole is not even in Antarctica.

Answer 2

If you are actually asking what would happen to the compass at the south magnetic pole, it would be pulled down sharply and point variously all over the place if the case is maneuvered to free up the needle.