NOTE: This problem has been fixed by Pixar and released in RenderMan 20.7. It was a interesting bug, so read on anyways!
While working on Shadows in the Grass I've been exploring building RenderMan shaders out of multiple PxrLMLayers. I've been having issues composing shaders of multiple translucent layers.
I've reduced it to a very simple case:
Example 1 (top stripe): I have a base diffuse material with an overlay. The base material is black, and the overlay is red with a gradient mask. Looks like I would expect.
Example 2 (middle stripe): I have a base diffuse material, and two overlays on top. The base material is black, the first overlay is green with a mask of 0, and the second overlay is red with a gradient mask.
I would expect the middle stripe to appear like the top stripe, but the green is somehow getting into the calculation despite its mask being zero.