aoe and casting healing

So I don't know if I'm right, but I have a problem with avoiding aoe despite the fact I'm out of their range longtime after they explode. It pisses me off a lot, if I stand with someone on the same radius and we both exit, I will take the dammage even if I stand on top of him, but he will not!
This only happens if I started to cast a spell (i'm white mage so healing one in that case ) when i'm inside an aoe. If I'm not casting I have 0 problem.
Does the game takes count about the fact my spell isn't finished and think I'm still here? I don't know the mechanic but it's frustrating and annoying to have to die a miserable die whenever my game isn't laggy and i'm doing my best like the rest of my group to avoid stupid death....
If it's coming from the game please fix it...