Arnold USD export: /mtl nodes not loaded when referencing USD in Maya USD Stage

Content:

I encountered an issue when exporting a USD file from Maya using Arnold. I would like to confirm if this behavior is expected.

Issue description:

Exported a USD file from Maya with Arnold materials. The material nodes are located under /mtl, which is at the same hierarchy level as the default prim /SC_jiedao_night_02_BG. The file is named sc.usd.

In Maya, I reference the USD file into a scene using Maya USD Stage:

file -reference "sc.usd";

After referencing, the geometry loads correctly, but the /mtl material nodes are not loaded, so materials are not applied or displayed.

Observations:

The USD file contains the /mtl prim with full material information.

The issue is not about material type (USD Preview Surface / MaterialX / Arnold), but rather that /mtl is at the same level as the default prim. Maya USD Stage appears to only load the default prim and its subtree.

Moving /mtl under the default prim allows the materials to be correctly referenced.

Possible cause:

Maya USD Stage reference mechanism seems to only load the default prim subtree, ignoring sibling prims.

Expected behavior / request:

Is there a way to have sibling /mtl nodes automatically loaded when referencing the USD, without restructuring the file?

Alternatively, could the Arnold USD export process automatically ensure material nodes are included under the default prim for proper referencing?

Any guidance or solution would be greatly appreciated.

Start by reproducing the Maya USD Stage reference command `file -reference "sc.usd";` and compare loading the default prim subtree with the sibling `/mtl` prim. Determine whether sibling materials can be included without restructuring `sc.usd`, or whether the Arnold export must place them under the default prim; done means a confirmed behavior and documented solution.

Written by the indexing model from the issue text.