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. 

The exporters could be incompatible with each other.
There are a lot of VRM exporters:
UniVRM (the official one, most complicated) - https://vrm.dev/en/how_to_make_vrm/
Blender VRM addon - https://github.com/saturday06/VRM_Addon_for_Blender
My own one - https://github.com/kitsune-ONE-team/KITSUNETSUKI-Asset-Tools

xbot model is actually from mixamo.com, it was exported using my own exporter. It can have it's own issues.

Most 3D artists are using VRoid . It directly exports models to VRM, but have messed up nodes tree and violates glTF specification. So I had to make some workarounds for it.

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.

You can post a link here, so I could check it out.

I can make some tools for exploring the contents of the VRM file for debugging, but for now the only thing that I have is this script:
https://github.com/kitsune-ONE-team/KITSUNETSUKI-Asset-Tools/blob/master/gltf-in...
It doesn't depends on anything so it could be downloaded and runned separately:
https://github.com/kitsune-ONE-team/KITSUNETSUKI-Asset-Tools/blob/master/kitsune...