Ok, the results of this test were very strange. I went through the xml and put a unique tile design in for each of the entries. I tried placing the entire set of 14 in the map editor in two groups.
For the first group, 2/14 took the custom tile. For the second group, 5/14 took the custom tile. So... I can't really make sense of it.
When I've started actual games with the xml in the mods folder, all shards seem to display the modified names, medallions, and descriptions, whether the tiles have taken the custom tile designs or not.