After the shield gets hit once, the blocking animation does not display. This happens for both First Person/Third Person views. In First Person View, the shield will look like a shield not being used. In Third Person View, the blocking animation changes.
Can confirm. Also applies to beds, conduits, and banners. and possibly other placeable objects with complex block models. However, the type of shading change is different for all the listed blocks, meaning sometimes it flips, or sometimes the shadow side gets halfway lighter, or even brighter than the lighter side.