The latest AMD Radeon drivers are bugging Blender 2.91.0 in the Viewport Shading-Material preview. Object is totally invisible.
Various reports have been made on the internet.
AMD and Blender have not yet responded on this while the problem has existed for quite some time.
Going back to a previous driver release solved the problem for me.
My Video Card is the:
AMD Radeon (TM) R9 390 Series