OpenUSD usdMtlx implicitly upgrades MaterialX documents on read or reference

[rajansldx]

When a MaterialX document authored in an older version is read through OpenUSD usdMtlx, the resulting USD reflects upgraded MaterialX version semantics even when no explicit conversion is requested.

Repro Steps:

• Unzip openusd_mtlx_auto_upgrade_repro.zip.
• Run: C:\usd-install\bin\usdcat.exe \atan2_sample.mtlx > \out.usda
• Open out.usda and confirm:

string config:mtlx:version = "1.39"
float inputs:inx = ...
float inputs:iny = ...

• Compare against input atan2_sample.mtlx:

Authored version is 1.38
Authored atan2 inputs are in1 / in2

Impact:

• Look preservation risk for published assets.
• Potential cross-site nondeterminism when different environments use different MaterialX versions.
• Unexpected node interface/schema changes during ingestion.

Expected:

• Default usdMtlx read behavior preserves authored MaterialX version semantics.
• Upgrade occurs only via explicit user/pipeline opt-in conversion.

Actual:

• Read/ingest path appears to apply upgrade behavior implicitly.

openusd_mtlx_auto_upgrade_repro.zip

[rajansldx]

https://academysoftwarefdn.slack.com/archives/C02HJH53RN3/p1768851405111839

[jesschimein]

Filed as internal issue #USD-12415

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