Materials in imported 3D asset are not in material library, "error rewriting document" when adding a material

Steps to reproduce:

• Create a 3D project
• Import the attached 3D asset
• Double click it in Component Library to go into component file
• -> Component's materials (red_material, blue_material and green_material) are not visible in Material Browser as expected
• Try creating a new material
• -> "Error rewriting document" dialog appears
• Looking at the file in text editor, it shows that materials are there but not under materialLibrary_. There also seems to be this "extra" material:
  DefaultMaterial { id: lambert1_material diffuseColor: "#ff666666"
  When this asset is imported to DS 3.4 I don't see this lambert1_material at all.