It looks like it's been learning that pieces have black borders, and since this pieceSet didn't, and it was a small part of the training set, it just fails and thinks we're looking at blank squares, more training data! From the label probabilities, it did a reasonable job of thinking the pieces were white, and their second best guesses tended to be close to the right answer, the blank spaces just won out.

Also, lets look at several random selections, including successes.