Unfortunately, there are multiple issues with UsdUtilsCreateNewUsdzPackage when it comes to layer referencing:
1) the function can't seem to properly export the referenced .mtlx file
2) even when inlining MaterialX as UsdShade, the referenced images are omitted
Unfortunately, there are multiple issues with UsdUtilsCreateNewUsdzPackage when it comes to layer referencing: 1) the function can't seem to properly export the referenced .mtlx file 2) even when inlining MaterialX as UsdShade, the referenced images are omitted
These two bug reports might be related: