I reloaded the save on the beach just to be sure and can confirm it was the harpies song that made the druid aggro. I wouldn't necessarily call it a bug, as every interaction worked as it should have (except maybe the druid attacking me instead of the harpy). I just think there should be a limit to how far a harpy can fly away. Or at least make the druid fight the harpy and not me, then go back to the ritual once the harpies are dead.
While it's a comical interaction there's nothing you can do to stop the slaying of multiple quest NPCs because of it, and I think that's the issue. Different then choosing a side (or not) and initiating the slaying yourself.
