When new material library node is created for a document that doesn't have it, all existing materials are automatically moved under the new material library node. The same should be done for existing textures as well.

To repro:

• import a 3D scene that has textured material(s) (e.g. Car.fbx)
• Edit imported component
• See how all the materials are moved under the newly created material library node but the textures aren't