For the sake of avoiding spoilers I will not say how to gain the mutation below.
While I was playing on a server I'm hosting with two friends I went through the necessary steps to gain the mutation Coup de Grass. The issue is my friends stepped away and had logged out during that leg of the game. These friends do not have the mutation and even when replicating the necessary steps to gain said mutation rendered no effect. I am hoping this is an unintentional bug because locking players out of mutations either because they were offline or because they joined part-way through a playthrough is not good.
I hope that someone from Obsidian can give me a better answer than the one I've already found which would be "Lose your tier III stuff and start a new game."
