Item frames randomly disappear.
To reproduce effect (Best done in Creative, although actual event happened in Survival):
1: Make a map wall with Item Frames and Maps 5 x 5.
2: Leave the chunks so they unload from memory.
3: Come back to chunk map wall is located in.
(If you don't get the same results the first time, repeat steps 2 and 3 until the result is accomplished.)

They also (once disappeared) come back if you leave the chunk the map wall used to be in so it unloads and come back. It should reappear.

Hope I could help!