I successfully installed 4.21.2 and converted my project. I then had the same issue with the material pack. After that I closed the project, deleted the folder and all of its contents (via file browser), and re-imported the asset pack via the Epic Games Launcher. I have the exact same issue. 3 of the 20 materials are visible in the pop-up menu on the blueprint actors, and any material, when dragged onto a simple static mesh in the world editor, does not show textures or normal maps.
Can you think of any other conflicts that could possibly be happening here?