It's worse than that. It also fails on this one:

(def board
  [[1 1 1 1 1 1 1 1 1]
   [1 1 1 1 1 1 1 1 1]
   [4 4 4 4 4 4 4 4 4]
   [4 4 4 4 4 4 4 4 4]
   [5 5 5 5 5 5 5 5 5]
   [6 6 6 6 6 6 6 6 6]
   [7 7 7 7 7 7 7 7 7]
   [8 8 8 8 8 8 8 8 8]
   [9 9 9 9 9 9 9 9 9]])