The shading applied to block_display entities is noticeably flat when compared to the shading applied to both blocks and the falling_block entity. Top faces appear brighter in general, and shading will occasionally go so far as to appear inverted, rendering darker on the top and lighter on the bottom where blocks would appear darker on the bottom and lighter on top. This is clearly not just some limitation on entity shading as, again, falling_block entities are shaded as expected.