It has come to my attention after a long period of testing that while the Spellcraft Bonus from the Practice of the Art Turn Undead variant for Magic Domain is active, Defense of the Arcane will never succeed its attempt. In effect this prevents Turn from being used while the Spellcraft bonus is active as there is a 1/3rd chance of wasting the attempt.
Sorry to disappoint, but not a bug. Turn Undead effects don't stack together.