I just lost two hours of play because last harpy won't move at all! It would just sing, making my character lost control.
And best part is that all you can is just sit there and relax, since you can't jump to reach harpy. And ranged weapons and spells? Sorry, dude, out of reach.

This would probably benefit from a screenshot or something. I've never seen any of the harpies go somewhere I couldn't get to pretty easily.

I lost my save (I simply reloaded and overwritten), but basically, harpy (only one was left), was hiding in bushes, several feet above ground, on highest cliff near the road, while I was at cove, way down. It was not moving, just singing and out of reach by at least ten turns, that much I would probably need to cross the searoad.

Try approaching the encounter with different tactics in mind.

Instead of bunching your party together in a narrow defile (I mean, it's a classic ambush scene right from the start, but you can see from the top of the path the options availible to you.) split them up, get ranged up on the paths, remember to use Lae'zels jump ability, or Misty Step, and some of the Illithid powers are useful here too.

I love this fight because it adds height into the equation, and the enemy is a master at the height advantage and more mobile than you so you have to fight smart.

For every part of the game: Explore, experiment (save often!), use tactics, and adapt.

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.

They generally won't stop singing if they have at least a majority of the party under control, however, the controlled party members should be getting saves each turn to break out of the song, and upon doing so get a normal turn. Were your characters getting their saves each turn?

yes, my characters are getting their save each turn

I've done that encounter on 7 separate occasions without any technical issues. Sure, it can be annoying if you get into a failed save loop, but at least one character will eventually snap out of it to progress the fight, and none of the harpies have ever gone anywhere that was out of reach, nor sing indefinitely. Not entirely clear on how you "lost two hours of play" either.

just go back the way you can and while facing in the direction of the druid grove, go left. Use jump spell, misty step,tadpole pull, pre setup your party, etc

you not reaching them is down to user error......you losing time "2hours" of time.... is down to user error.