Did some extensive testing and discovered that the reason all my models were not loading on mobile (app or browswer on iOS) was because all my models were assigned Maya’s StingrayPBS material type.
Instead, if I assign them a Lambert material, then manually upload all my textures and assign them in the Sketchfab viewer interface, they will load on mobile.
(FWIW, I also tried manually loading the textures AND using the StingrayPBS material, but that yielded the same broken mobile results. And I even tried using a StingrayPBS material with NO textures assigned. Same problem.)
Since the StingrayPBS material type works on desktop, is there any chance this could get fixed for mobile? I’d really prefer not to have to re-upload every single one of my models…