Map markers only show the most recently placed map

Maps will only show a green marker for the last map that was placed in an item frame.

Steps to reproduce:
1) Create a new map
2) Make some copies
3) Place one copy in an item frame on a wall
4) Walk a short distance and place the other copy in a second item frame

Expected result: Two green markers on the map

Actual result: The first green marker disappears, and the only marker visible is the one for the map you just placed. You can go and break the first map and put it back on the wall and its marker will appear, but the marker from the second map will then vanish. Placing any additional copies will also only show the marker for the copy you just placed, while the others are no longer visible.