I'm not sure that's actually a bug/pretty sure that's by design. I don't know this for a fact, but with FFXI spell interruption wasn't something that guaranteed, it was like a percentage change. At low levels anything you did would interrupt a spell, but at higher levels with different gear you could get away with movements/damage without a spell being interrupted.

I haven't seen any "negative" spell interruption gear for FFXIV yet although I'm assuming some will existing at some point. I know that on my WHM on FFXI I had so much negative spell interuption gear I could almost run around casting (unless a large monster was directly attacking me, I could get away with just about any spells).

I could be completely wrong of course since my theory is all based on FFXI, but I just feel that interruption is still a chance, not a guarantee. When you get attacked, sometimes you get interrupted whilst casting, sometimes you don't.