Same thing happened to me in 2 separate playthroughs but at a different point in act 2.
The harpers/fist/jaheira turn hostile right after the battle on ground floor of moonrise towers if any of them is taking even one damage from environmental effects such as fire or electrocution which is VERY likely to happen after a battle with so many enemies and spellcasters such as this one.
Very annoying

This just happened to me as well. At the base of the moonrise tower, the harpers walked right into a plague swarm after the battle and instantly threw Shadowheart in prison because paying didn't work. Hilariously, they put her in the Moonrise dungeon even though the tower was still under control of Ketheric. Instead of doing it all over again, I managed to sneak out of the dungeon with Shadowheart and get her back to my party, but it was quite annoying.
They will also become hostile if I go into another room upstairs in the tower even though it's not under their control. Super weird bugs.