White, Stockfish, to move. It plays bb6-g1,losing the bishop and the win.
Also strange: the same engine does not make this weird move in the Tarrasch program. Playing with KBN against K, I've seen Stockfish often blunder away the advantage in these endgame setups.
Any clue how this could be happening, or why it would happen only with Arena?
EDIT: SmallChess answer is correct. The problem was a residual, non-zero value in the position "number of half-moves" setting. See my reply to his posting for more information.