Hi, I am having the an issue with USD and MTLX. I have a feeling that I might be misunderstanding how unreal is reading the files. I can only get my mtlx materials to show black. This happens with both a standalone mtlx file and one within a usd. My files appear correctly in Maya and Solaris.
Which version of the engine are you using?
Would it be possible to share the sample with us?
AFAIK UE only support the version with mtlx file as standalone.