I saw a post made 18 days ago about the same issue, but with USD.
This is a serious bug, as UDIM a very much needed for Productions.
I’ve written my own exporter/importer for MaterialX for Both Maya and Blender.
Had I known of this issue, I would have instead built an importer for Unreal, forgoing MatX completly 
… This would have been more flexible too 
I guess I’ll have to look into that.