There are two enchantment glint textures, one for items and one for entities. The shield in the inventory uses the item glint instead of the entity glint. However, the same shield in all other contexts uses the entity glint, as well as shields with banners applied to them in all contexts. This leads me to believe that this is a bug due to the inconsistency.

Expected behavior: All display contexts for the shield use the same enchantment glint texture
Observed behavior: shield in the inventory uses the item glint while other display contexts use the entity glint