Scenario: you have UDIMs 1001, 1011, 1012 in Substance Painter, but you tick only 1001 for exporting to GLTF. You then choose to upload all materials in the GLB/GLTF file in the SL Viewer. The file still has information about the other tiles, so the viewer creates Material assets for all of the UDIM tiles, even though only 1001 has textures to go along with it. It shouldn't have created these empty assets.
What needs fixing: The Material uploader need to check for texture data for each material, if there is none it should not create Material assets for it in the inventory. Ideally, it should not offer to upload them even.