I have finished the sprang part. As I start the next woven section, this ripple effect is appearing where the draw-in of the sprang is much more than that of the plain weave. It is only showing up at each edge of the fabric, but I am hoping that when the whole thing comes off the loom and gets wet-finished, it will do this across the entire width of the cloth.
Here is the finished sprang section. The top and bottom are interlaced sprang, and the middle section is interlinked sprang, completing my "cartouche" shape.