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}