Is this a bug? See if you can view this AR file (of the Mac Pro). It works fine on my iPhone with iOS 12. On the iPad it comes up blank with the text “Preview Universal Scene Description (Mobile)”.
https://www.apple.com/105/media/us/...41c-9a5b-349c6365bb36/quick-look/case-on.usdz
https://www.apple.com/105/media/us/...41c-9a5b-349c6365bb36/quick-look/case-on.usdz