What I expected to happen was...:
Maps rendering consistently under the same lighting (15 sky, 0 block (block didn't seem to affect the issue)), rotation and angle of observation.
 
What actually happened was...:
Instead maps would flicker seemingly randomly, walking/turning camera infront of them would make some appear sometimes dark and then bright again and the icons (burried treasure X or green arrow tested) were sometimes dark.
 
Steps to Reproduce:
1. Get Burried Treasure Map, normal map (possibly the other ones too)
2. Open them (you have to with the treasure maps, with normal maps it wouldn't make sense otherwise) and place them into item frame
3. Rotate them, walk and look around (I recommend a little wall for convinience)
4. Sidenote for reproduction: "empty" (noisy background...) treasure maps showed the same behavior.
 
Btw.: Yes, I stole the template from the guideline.