I don't think tactics are an issue here. The question is whether or not this is a bug, not how to win the fight.

It may be that the harpy is staying up there because that's where it wants to be, in which case it's probably not a bug. But some behavioral changes for the harpies might be something to consider if the current model frequently leads to an unsatisfying fight (particularly given that you can't see the harpies until you trigger the encounter, so you will probably be grouped up down at the water when it starts). I haven't personally had issues with any of the harpies camping out in places that are too difficult to reach - they're usually singing a little bit and then coming after me.