NPCs Do Not Fight During Varnik Battle

I am aiming for Merciful achievement and so far I managed to do pretty much every quest and encounter without killing anyone. Now I have finally got to the Battle of Vranik. The army runs towards the gate and batters it open and soldiers pour in. And then no fighting is happening. The NPCs are just circling around each other not fighting at all. I can freely join in and hack everyone to pieces without any reaction from the NPCs of course but that would obviously void the Merciful achievement (some 100 hours in the game at this point). Is this a known bug? How can I overcome this? I am using no mods. Thanks!

Also knocking them out can be done but I still need my guys to do the finishing blow for each one of them since I don’t have a horse there to exploit the fact that running (and killing) people on a horse does not void the achivement.

I have discovered a workaround. Loading an earlier save (or autosave) befor all those lenghty cutscenes and conversations and going from there fixes the problem. The issue therefore lies with the autosave that is created at the start of that battle.