This isnt a bug-- SPOILER: Asterion is a vampire, running water hurts him.
I suspected that running water (why just normal spring water more or less is beyond me) would do something to him but the threshold for being by it or out of it is not clearly/visually communicated.
Bug or not, he is basically useless in that entire zone. You can only tell that he is in water, visually, when he takes a step and you see ripples in his steps. Otherwise you have no idea that he's still in water.
