Only the "filez" texture is used by Mtlx Triplanar Projection

[TomMinor]

Describe the bug
If I plug in 3 textures to the triplanar node

I would expect this

This works as expected when using kick, but fails in the viewport (only a single texture is used)

To Reproduce
Steps to reproduce the behavior:

1. Create a Mtlx Triplanar Node
2. Plug in 3 textures
3. Observe as only the last texture is used

Expected behavior
All 3 textures should be used.

Example USD file
unhappy_triplanar_shader.zip

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-13587