As you can see in this screenshot https://i.imgur.com/NtujMds.png it's supposed to take the damage instead of my core argument. I had just paid a character 50 shills for that, so it should do something.
Previously, the same thing happened when arguing with a different character earlier in the run's day. I paid 2 people to help with the negotiation but it did nothing. I closed the game and re-opened it, effectively resetting the negotiation, and that's when it worked as intended.
I just tried closing the game again and it actually worked https://i.imgur.com/6QaFFo6.png
It is kind of annoying. I'd rather keep my money if it wasn't going to do anything.
Hired Brill, civilian laborer, by giving him 50 shills to work for me. Headed out to meet Fellemo in the first day, and when trying to convince Arint that I'm telling the truth, Brill joins the negotiation but is useless. Closed the game, resumed, restarted the negotiation, and it worked. https://i.imgur.com/6QaFFo6.png