Borworn,
Is this problem still occurring?
I do see a few defects against this sort functionality but nothing specifically matching what you report here.
Though I did test and see the same behavior in a 2.4.1 lab system.
Further testing on a new 2.5 lab system shows I am no longer able to reproduce this. Each column works just fine when choosing to sort it by Ascending or Descending order.
Thanks,
Mike