I think what will have to be done is to test using Blitzstrahl (4.5 second cast with stun effect) on a mob like an elemental that can use Ancient Magic. Hopefully get a few very clear instances of Blitzstrahl start > AM start > Blitzstrahl land > stun wear > AM land/interrupt. The main thing that bothers me is the idea that the time the cast starts should have anything to do with the stun effect, glitch or not. You should be able to start casting a 30-second stun cast and if it happens to land while a mob is casting it should stun it, there's no reason it would be programmed otherwise.
I guess another possibility, if not just chatlog confusion where the stun had already worn as I suggested above, would be that the stun lands at the exact same time as the mob chooses to start casting, so the game doesn't break the tie and just lets both go ahead.
XI Wiki


