I asked K. M. Weiland, “In the flat ARC, is there a ‘dark night of the soul’?”
Her gracious reply:
Thanks for your note! In storytelling terms, I equate the Dark Night of the Soul with the Low Moment at the Third Plot Point. This will occur in all types of arcs (to varying degrees), and the Flat Arc is no difference. I talk about that in the book in the section about the Flat Arc’s Third Plot Point:
“In a flat character arc, the protagonist won’t actually doubt the Truth, but he will be brought to a point where he seriously doubts his ability to use the Truth to defeat the Lie.”
In regard to the Third Plot Point in general, you might also find these posts interesting:
helpingwritersbecomeauthors. com/the-link-between-your- storys-first-plot-point-and- third-plot-point/
All the best!