Below are links to pictures I took last night of the saddle girthed up on his back but without the pad underneath. (Sorry if they are not fantastic. It was dark and I was by myself and he was fidgety because he wanted to go back out in the pasture and eat.) I felt around underneath it as best I could, and the panels seemed to be making good contact with his back. It also has a nice roomy channel over his spine.
The saddle was brand new in January and fit him really well at the time. (The owner of the saddle shop where I bought, Lynnda at Classic Saddlery, it is an experienced fitter and helped me fit it to him according to the wither tracing I took.) During August and September I didn't ride much because when it wasn't super hot I was helping some friends with their fence project, but a friend was still using him for the occasional beginner lesson. I am really hoping this is not a saddle fit issue!