When you view the models in Xcode or Finder do the materials appear correctly?
Are you converting them yourself? If so what model type are you converting from?
Are you able to upload one of the files for someone here to investigate or are they propriatory files you don't want to share?
If you converted them yourself and so you have a good glb/fbx etc file, try using the latest usdzconvert tool and see if the problem persists.
We have a similar problem, models work fine in ios 12 but updated to ios 13 and we get strange things happening to the models.
What was the problem, Chris? I'm seeing an issue where smoothing isn't being applied.