Is the heading used near the pole?
Near the poles 'polar grid navigation' is used. By placing a rectangular grid over the pole, aligned with the prime meridian, a local coordinate system is created. Aircraft fly constant grid courses near the poles. 

For your example crossing the south pole approaching over the prime meridian the true course will change from 180° (T) to 000° (T), but the grid course will remain 180° (G) all the time. 

Grid courses and true courses can be converted easily, this youtube video explains it nicely (although he gets the grid track in the wrong direction in the end, it should be 110°(G) instead of 290°(G))