This has happened to me exactly once. If you remember how the Death Claws in add groups attach purple lines to the boss, it's these that provide the massive defense buff. Ordinarily, Cid destroys the death claws and this removes the buff. However, there was one time when Cid destroyed the claws, the lines disappeared... but the buff didn't go away! We ended up wiping on purpose to reset the fight and it didn't happen again the next time. Definitely an odd glitch.