I haven't ran BLM all that much but on SCH I can turn my back on someone and still cure them (Firing cures at both tanks during T1 for example). Do magic spells on monsters act differently? If so I haven't noticed it on SMN either. Do you have the option checked to face your target? That could be your problem. Without that checked I have to physically turn my DRG or I can't attack for lack of LoS. An enemy dying but your CD being eaten is unfortunately inevitable in some situations since the monster being dead signal can very easily reach your client AFTER the signal reaches the server that your cast finished. Even at low latencies the delay from the signal going back and forth from the server can be enough to make these events happen out of order and eat your CD as a result.