I can confirm that this bug is still present in 6.5 in the Steam version. Being unable to complete the game because of a bug is a very dissatisfying ending to an otherwise great gameplay experience. I know I could beat this scenario without using the Quill, but doing so would require a lot of luck and grinding, which I have no desire to do.
This game is amazing in general, and I think it is a shame the developers haven't had the time to fix bugs like this.