This is not a duplicate of MC-605, MC-31619, or MC-35848. Those three bugs are fixed, and are actually unrelated to this. This bug occurs in 1.7.2, the latest version at the time of this writing.

What I expected to happen:
Maps placed in item frames would be shaded the same as the item frames themselves are.

What actually happened:
When a map is placed in an item frame, it will only shade properly if you have the GUI hidden via F1. Even then, it may not always be shaded. in some circumstances. For me, hiding the GUI seems to be the most effective way to get shading to happen, but view angle is also a factor in whether it works. Your results may vary; the instructions below are how I've been able to observe it.

Steps to Reproduce:

1. Place item frames on walls facing different directions.
2. Observe that the frames are shaded to the same degree as the block face they are attached to.
3. Place maps in the frames. Have your GUI visible.
4. Observe that the maps are not shaded.
5. Press F1 to hide your GUI.
   • You may have to walk around, rotate your camera, or reload the world before the maps look different.
6. Observe that the maps are now shaded properly.
7. Press F1 to show the GUI again.
8. Observe that the maps are no longer shaded.