The bug

Maps in item frames (both regular and glow item frames) being tested by comparators will output 1,2,3,4 for the four orientations a map can have on an item frame, but will then output 5,6,7,8 for those same orientations when rotated a second time.

Steps to reproduce

1. Place an item frame, and a comparator that reads from it. Place a map (already 'used,' with some drawing that is not rotationally symmetrical) inside it. The map will expand to fill the edge, as expected.
2. Rotate the map 3 times; the comparator will output signals with respective strengths of 1, 2, 3, and 4, as expected.
3. Rotate the map again. The map is now back to an orientation identical to the original orientation of the map, yet the comparator now outputs 5. Continuing rotation of the map will step through 6, 7, and 8.

Code analysis

Code analysis by HiroTDK can be found in this comment.