Just following up. There was an error in the UVTT file exported from Dungeon Draft that improperly specified the map_size property. It also had a truncated version of the map image. If anyone runs across that again, the fix is to edit the map_size property in the file to match the size of the map image. In this case it was changing this: "map_size":{"x":21,"y":28} to this: "map_size":{"x":50,"y":50}