One fix was added here: https://kindeyegames.itch.io/c3-3dobject-alpha/devlog/378121/fix-glb-import-fix-...
This should make the glb files work (also thanks to Zaykoworks for the additional idea for a workaround.)
In the above post, I also noted why the original gltf files do not currently work, so please continue using the converted glb versions for now. I now also have a good example Blockbench gltf file to work with to see if I can support it.