Hi, we have experienced this issue sometimes on iOS Safari (14.0.1 Macbook M1 Pro) and on an Android phone (Fairphone 3 / Android 10 / Chrome).
It disappears and reappears randomly by reloading the browser tab. Also for the main "astronaut.glb" example on your example site it's the same. I tried to add directional lights to our 3D model, this addds a third state:
1.) Normal perfect evenly lit. 2.) Unlit completely black 3.) Directional lit, a little bit, our custom changes.
If you reload the browser tab you can experience the states jumping randomly. As a side fact, I read out any modelviewer details like exposure, materials, etc. They seem to be identically, so there is no correlation with a bad setting or anything like this. I think there is some problem "under the hood".
Thanks for your help! Stefan