There is a wait between the time monsters start casting a spell and the time that they finish casting, just like with players. So what happens is, they started casting a spell (though you don't know it because they don't have words of power over their heads), you paralyzed them, and then they finished casting and blasted you or teleported. It's super incredibly annoying when this happens and a spell bounces off your reflect and frees them from the Paralyze. Monsters spells can't be interrupted fyi. They can cast through anything. Also, they precast and hold spells to blast you with when they're chasing you. That's why when you break LOS and then enter it again they'll instantly blast you with a spell when you're in range.
Dragons/mares can firebreath while paralyzed, but if you weaken them to about 10%hp the damage is almost non-existant.
