Prototypes are randomly assigned different shaders

**Describe the bug**
With a scene that has both nested instanceable references and nested point instancer primitives, the instances are rendered with different colors on some frames. Both with husk and kick. Or if you kick the usda file over and over, sometimes you get different colors.

**To Reproduce**
Steps to reproduce the behavior:
1. Load the attached usda into Houdini as a sublayer and render frames 1-12
2. Or, kick the usda file and render the 12 frames

**Used Software Versions**
- Arnold 7.4.2
- Houdini 20.5.584

**Additional context**
I resaved the 12 frames as ass files, and in some ass files, different shaders were assigned to a polymesh.

Start by loading the attached USDA as a Houdini sublayer and rendering frames 1–12, then repeat the same render with kick and compare shader assignments across runs. Trace the nested instanceable references and nested point instancer primitives involved in the generated ASS files; done means identical shader assignments and colors on every frame and repeated render.

Written by the indexing model from the issue text.