The supplied xbot.vrm seems to work consistently. When it's not working, it seems like there's some issue with grabbing, or starting the camera.
I have tried to use other models.
One which I found a more "full" model that I could download (legally, though really only done for testing purposes before looking at paying for one), which ended up deeply skewed. But the geometry seemed to be there/displayed.
Another after trying to edit the xbot.vrm through Blender, and exporting it as a vrm. This ended up with missing geometry/undisplayed (transparent) geometry.
Another where I imported and re-exported the xbot.vrm with the same issue with the "missing geometry".
I'm not sure where to go in order to upgrade from that working model, to be able to figure out why other ones are not working.