Thanks for reporting this.
Our Blender processing is based on scripts and Blender features are sometimes blocked/skipped during script execution (for security purpose), so that could explain the result you have on Sketchfab. To reproduce the issue in Blender, simply open your Blend file and without clicking "Reload Trusted", select another object of your scene to update the viewport and you should see your mesh broken.
In fact, I don't really know how it works when you open it in the same Blender than the one that served to create the scene. Maybe you will not have the warning and the "Reload Trusted" button, or maybe you enabled the option that allows it to be loaded automatically.
Anyway, here is my result right after having opened your file and selected an object to trigger the viewport update.
Our scripts don't unlock these features for now, for security purpose (you could imagine a .blend file containing a python scripts which is run at the opening and can perform "very bad things" with the filesystem)
The issue has been logged a few months ago on our side, and I will look for a solution when I will have time
Thanks! I hope it helps